<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-02-24T16:43:22.0335375+00:00 -->
<!-- Version            : 5.0.1.571 -->
<!-- Package ID         : 19fb4f4ba39c4e5496fc22bf5dabd4ad -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
<xsd:schema xmlns:nonnum="http://www.xbrl.org/dtr/type/non-numeric" xmlns:dei="http://xbrl.sec.gov/dei/2020-01-31" xmlns:num="http://www.xbrl.org/dtr/type/numeric" xmlns:us-types="http://fasb.org/us-types/2020-01-31" xmlns:stpr="http://xbrl.sec.gov/stpr/2018-01-31" xmlns:srt-types="http://fasb.org/srt-types/2020-01-31" xmlns:ctmx="http://cytomx.com/20201231" xmlns:sic="http://xbrl.sec.gov/sic/2020-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:srt="http://fasb.org/srt/2020-01-31" xmlns:us-gaap="http://fasb.org/us-gaap/2020-01-31" xmlns:naics="http://xbrl.sec.gov/naics/2017-01-31" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:country="http://xbrl.sec.gov/country/2020-01-31" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:currency="http://xbrl.sec.gov/currency/2020-01-31" xmlns:exch="http://xbrl.sec.gov/exch/2020-01-31" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://cytomx.com/20201231" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:import schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/instance" />
  <xsd:import schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/linkbase" />
  <xsd:import schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" namespace="http://xbrl.org/2005/xbrldt" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd" namespace="http://xbrl.sec.gov/country/2020-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/currency/2020/currency-2020-01-31.xsd" namespace="http://xbrl.sec.gov/currency/2020-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd" namespace="http://xbrl.sec.gov/dei/2020-01-31" />
  <xsd:import schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd" namespace="http://www.xbrl.org/dtr/type/non-numeric" />
  <xsd:import schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd" namespace="http://www.xbrl.org/dtr/type/numeric" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/exch/2020/exch-2020-01-31.xsd" namespace="http://xbrl.sec.gov/exch/2020-01-31" />
  <xsd:import schemaLocation="http://www.xbrl.org/lrr/arcrole/factExplanatory-2009-12-16.xsd" namespace="http://www.xbrl.org/2009/arcrole/fact-explanatoryFact" />
  <xsd:import schemaLocation="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd" namespace="http://www.xbrl.org/2009/role/negated" />
  <xsd:import schemaLocation="http://www.xbrl.org/lrr/role/net-2009-12-16.xsd" namespace="http://www.xbrl.org/2009/role/net" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/naics/2017/naics-2017-01-31.xsd" namespace="http://xbrl.sec.gov/naics/2017-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/sic/2020/sic-2020-01-31.xsd" namespace="http://xbrl.sec.gov/sic/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd" namespace="http://fasb.org/srt/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-roles-2020-01-31.xsd" namespace="http://fasb.org/srt-roles/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/srt/2020/elts/srt-types-2020-01-31.xsd" namespace="http://fasb.org/srt-types/2020-01-31" />
  <xsd:import schemaLocation="https://xbrl.sec.gov/stpr/2018/stpr-2018-01-31.xsd" namespace="http://xbrl.sec.gov/stpr/2018-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd" namespace="http://fasb.org/us-gaap/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-roles-2020-01-31.xsd" namespace="http://fasb.org/us-roles/2020-01-31" />
  <xsd:import schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-types-2020-01-31.xsd" namespace="http://fasb.org/us-types/2020-01-31" />
  <xsd:annotation>
    <xsd:appinfo>
      <link:roleType id="TemplateLink" roleURI="http://cytomx.com/20201231/role/TemplateLink" xmlns:link="http://www.xbrl.org/2003/linkbase">
        <link:definition>00000 - Document - Template Link</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="ctmx-20201231_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:title="Calculation Links, all" xlink:type="simple" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="ctmx-20201231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:title="Definition Links, all" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="ctmx-20201231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:title="Label Links, all" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="ctmx-20201231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:title="Presentation Links, all" xlink:type="simple" />
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DocumentDocumentAndEntityInformation" id="Role_DocumentDocumentAndEntityInformation">
        <link:definition>100000 - Document - Document and Entity Information</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/StatementBALANCESHEETS" id="Role_StatementBALANCESHEETS">
        <link:definition>100010 - Statement - BALANCE SHEETS</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/StatementBALANCESHEETSParenthetical" id="Role_StatementBALANCESHEETSParenthetical">
        <link:definition>100020 - Statement - BALANCE SHEETS (Parenthetical)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/StatementSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS" id="Role_StatementSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS">
        <link:definition>100030 - Statement - STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/StatementSTATEMENTSOFSTOCKHOLDERSEQUITY" id="Role_StatementSTATEMENTSOFSTOCKHOLDERSEQUITY">
        <link:definition>100040 - Statement - STATEMENTS OF STOCKHOLDERS&apos; EQUITY</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/StatementSTATEMENTSOFCASHFLOWS" id="Role_StatementSTATEMENTSOFCASHFLOWS">
        <link:definition>100050 - Statement - STATEMENTS OF CASH FLOWS</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureDescriptionOfBusiness" id="Role_DisclosureDescriptionOfBusiness">
        <link:definition>100060 - Disclosure - Description of the Business</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies">
        <link:definition>100070 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureAdoptedAndRecentAccountingPronouncements" id="Role_DisclosureAdoptedAndRecentAccountingPronouncements">
        <link:definition>100080 - Disclosure - Adopted and Recent Accounting Pronouncements</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureNetLossPerShare" id="Role_DisclosureNetLossPerShare">
        <link:definition>100090 - Disclosure - Net Loss Per Share</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureFairValueMeasurementsAndShortTermInvestments" id="Role_DisclosureFairValueMeasurementsAndShortTermInvestments">
        <link:definition>100100 - Disclosure - Fair Value Measurements and Short-Term Investments</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipment" id="Role_DisclosurePropertyAndEquipment">
        <link:definition>100110 - Disclosure - Property and Equipment</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureGoodwillAndIntangibleAssets" id="Role_DisclosureGoodwillAndIntangibleAssets">
        <link:definition>100120 - Disclosure - Goodwill and Intangible Assets</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureAccruedLiabilities" id="Role_DisclosureAccruedLiabilities">
        <link:definition>100130 - Disclosure - Accrued Liabilities</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreements" id="Role_DisclosureResearchAndCollaborationAgreements">
        <link:definition>100140 - Disclosure - Research and Collaboration Agreements</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLicenseAgreement" id="Role_DisclosureLicenseAgreement">
        <link:definition>100150 - Disclosure - License Agreement</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureCommitmentsAndContingencies" id="Role_DisclosureCommitmentsAndContingencies">
        <link:definition>100160 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLeases" id="Role_DisclosureLeases">
        <link:definition>100170 - Disclosure - Leases</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureCommonStock" id="Role_DisclosureCommonStock">
        <link:definition>100180 - Disclosure - Common Stock</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureStockBasedCompensation" id="Role_DisclosureStockBasedCompensation">
        <link:definition>100190 - Disclosure - Stock-based Compensation</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxes" id="Role_DisclosureIncomeTaxes">
        <link:definition>100200 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureDefinedContributionPlan" id="Role_DisclosureDefinedContributionPlan">
        <link:definition>100210 - Disclosure - Defined Contribution Plan</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureSubsequentEvents" id="Role_DisclosureSubsequentEvents">
        <link:definition>100220 - Disclosure - Subsequent Events</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies">
        <link:definition>100230 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesTables" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesTables">
        <link:definition>100240 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureNetLossPerShareTables" id="Role_DisclosureNetLossPerShareTables">
        <link:definition>100250 - Disclosure - Net Loss Per Share (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureFairValueMeasurementsAndShortTermInvestmentsTables" id="Role_DisclosureFairValueMeasurementsAndShortTermInvestmentsTables">
        <link:definition>100260 - Disclosure - Fair Value Measurements and Short-Term Investments (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentTables" id="Role_DisclosurePropertyAndEquipmentTables">
        <link:definition>100270 - Disclosure - Property and Equipment (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureGoodwillAndIntangibleAssetsTables" id="Role_DisclosureGoodwillAndIntangibleAssetsTables">
        <link:definition>100280 - Disclosure - Goodwill and Intangible Assets (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureAccruedLiabilitiesTables" id="Role_DisclosureAccruedLiabilitiesTables">
        <link:definition>100290 - Disclosure - Accrued Liabilities (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsTables" id="Role_DisclosureResearchAndCollaborationAgreementsTables">
        <link:definition>100300 - Disclosure -  Research and Collaboration Agreements (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLeasesTables" id="Role_DisclosureLeasesTables">
        <link:definition>100310 - Disclosure - Leases (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureCommonStockTables" id="Role_DisclosureCommonStockTables">
        <link:definition>100320 - Disclosure - Common Stock (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureStockBasedCompensationTables" id="Role_DisclosureStockBasedCompensationTables">
        <link:definition>100330 - Disclosure - Stock-based Compensation (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxesTables" id="Role_DisclosureIncomeTaxesTables">
        <link:definition>100340 - Disclosure - Income Taxes (Tables)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesScheduleOfCustomersAndCollaborationPartnersWhoRepresent10OrMoreOfTotalRevenueDuringEachPeriodPresentedOrNetAccountsReceivableBalanceDetails" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesScheduleOfCustomersAndCollaborationPartnersWhoRepresent10OrMoreOfTotalRevenueDuringEachPeriodPresentedOrNetAccountsReceivableBalanceDetails">
        <link:definition>100350 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesAdditionalInformationDetails" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesAdditionalInformationDetails">
        <link:definition>100360 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesSummaryOfReconciliationOfCashCashEquivalentsAndRestrictedCashDetails" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesSummaryOfReconciliationOfCashCashEquivalentsAndRestrictedCashDetails">
        <link:definition>100370 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesScheduleOfUsefulLivesOfPropertyAndEquipmentDetails" id="Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesScheduleOfUsefulLivesOfPropertyAndEquipmentDetails">
        <link:definition>100380 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureAdoptedAndRecentAccountingPronouncementsAdditionalInformationDetails" id="Role_DisclosureAdoptedAndRecentAccountingPronouncementsAdditionalInformationDetails">
        <link:definition>100390 - Disclosure - Adopted and Recent Accounting Pronouncements - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureNetLossPerShareSummaryOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedNetLossPerShareDetails" id="Role_DisclosureNetLossPerShareSummaryOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedNetLossPerShareDetails">
        <link:definition>100400 - Disclosure - Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureFairValueMeasurementsAndShortTermInvestmentsScheduleOfShortTermInvestmentsSubjectToFairValueMeasurementsOnRecurringBasisDetails" id="Role_DisclosureFairValueMeasurementsAndShortTermInvestmentsScheduleOfShortTermInvestmentsSubjectToFairValueMeasurementsOnRecurringBasisDetails">
        <link:definition>100410 - Disclosure - Fair Value Measurements and Short-Term Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureFairValueMeasurementsAndShortTermInvestmentsAdditionalInformationDetails" id="Role_DisclosureFairValueMeasurementsAndShortTermInvestmentsAdditionalInformationDetails">
        <link:definition>100420 - Disclosure - Fair Value Measurements and Short-Term Investments - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentSummaryOfPropertyAndEquipmentNetDetails" id="Role_DisclosurePropertyAndEquipmentSummaryOfPropertyAndEquipmentNetDetails">
        <link:definition>100430 - Disclosure - Property and Equipment - Summary of Property and Equipment, Net (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosurePropertyAndEquipmentAdditionalInformationDetails" id="Role_DisclosurePropertyAndEquipmentAdditionalInformationDetails">
        <link:definition>100440 - Disclosure - Property and Equipment - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureGoodwillAndIntangibleAssetsAdditionalInformationDetails" id="Role_DisclosureGoodwillAndIntangibleAssetsAdditionalInformationDetails">
        <link:definition>100450 - Disclosure - Goodwill and Intangible Assets - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureGoodwillAndIntangibleAssetsScheduleOfGoodwillAndIntangibleAssetsDetails" id="Role_DisclosureGoodwillAndIntangibleAssetsScheduleOfGoodwillAndIntangibleAssetsDetails">
        <link:definition>100460 - Disclosure - Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureAccruedLiabilitiesSummaryOfAccruedLiabilitiesDetails" id="Role_DisclosureAccruedLiabilitiesSummaryOfAccruedLiabilitiesDetails">
        <link:definition>100470 - Disclosure - Accrued Liabilities - Summary of Accrued Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsScheduleOfRevenueByCollaborationPartnersDetails" id="Role_DisclosureResearchAndCollaborationAgreementsScheduleOfRevenueByCollaborationPartnersDetails">
        <link:definition>100480 - Disclosure - Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails">
        <link:definition>100490 - Disclosure - Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails1" id="Role_DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails1">
        <link:definition>100500 - Disclosure - Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails1Default" id="Role_DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails1Default">
        <link:definition>100500 - Disclosure - Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details 1)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsAmgenIncAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsAmgenIncAdditionalInformationDetails">
        <link:definition>100510 - Disclosure - Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsAstellasPharmaIncAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsAstellasPharmaIncAdditionalInformationDetails">
        <link:definition>100520 - Disclosure - Research and Collaboration Agreements - Astellas Pharma Inc - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsBristolMyersSquibbCompanyAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsBristolMyersSquibbCompanyAdditionalInformationDetails">
        <link:definition>100530 - Disclosure - Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsImmunogenIncAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsImmunogenIncAdditionalInformationDetails">
        <link:definition>100540 - Disclosure - Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsPfizerIncAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsPfizerIncAdditionalInformationDetails">
        <link:definition>100550 - Disclosure - Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsSummaryOfContractLiabilitiesDetails" id="Role_DisclosureResearchAndCollaborationAgreementsSummaryOfContractLiabilitiesDetails">
        <link:definition>100560 - Disclosure - Research and Collaboration Agreements - Summary of Contract Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureResearchAndCollaborationAgreementsContractLiabilitiesAdditionalInformationDetails" id="Role_DisclosureResearchAndCollaborationAgreementsContractLiabilitiesAdditionalInformationDetails">
        <link:definition>100570 - Disclosure - Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLicenseAgreementAdditionalInformationDetails" id="Role_DisclosureLicenseAgreementAdditionalInformationDetails">
        <link:definition>100580 - Disclosure - License Agreement - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails" id="Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetails">
        <link:definition>100590 - Disclosure - Commitments and Contingencies - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLeasesAdditionalInformationDetails" id="Role_DisclosureLeasesAdditionalInformationDetails">
        <link:definition>100600 - Disclosure - Leases - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLeasesSummaryOfSupplementalInformationRelatedToLeasesDetails" id="Role_DisclosureLeasesSummaryOfSupplementalInformationRelatedToLeasesDetails">
        <link:definition>100610 - Disclosure - Leases - Summary of Supplemental Information Related to Leases (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLeasesScheduleOfMaturityOfOperatingLeaseLiabilitiesDetails" id="Role_DisclosureLeasesScheduleOfMaturityOfOperatingLeaseLiabilitiesDetails">
        <link:definition>100620 - Disclosure - Leases - Schedule of Maturity of Operating Lease Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureLeasesScheduleOfMaturityOfOperatingLeaseLiabilitiesDetails2" id="Role_DisclosureLeasesScheduleOfMaturityOfOperatingLeaseLiabilitiesDetails2">
        <link:definition>100630 - Disclosure - Leases - Schedule of Maturity of Operating Lease Liabilities (Details)2</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureCommonStockAdditionalInformationDetails" id="Role_DisclosureCommonStockAdditionalInformationDetails">
        <link:definition>100640 - Disclosure - Common Stock - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureCommonStockReservedSharesOfCommonStockIssuanceOnConvertedBasisDetails" id="Role_DisclosureCommonStockReservedSharesOfCommonStockIssuanceOnConvertedBasisDetails">
        <link:definition>100650 - Disclosure - Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureStockBasedCompensationAdditionalInformationDetails" id="Role_DisclosureStockBasedCompensationAdditionalInformationDetails">
        <link:definition>100660 - Disclosure - Stock-based Compensation - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureStockBasedCompensationScheduleOfActivityUnderCompanySStockOptionPlansDetails" id="Role_DisclosureStockBasedCompensationScheduleOfActivityUnderCompanySStockOptionPlansDetails">
        <link:definition>100670 - Disclosure - Stock-based Compensation - Schedule of Activity Under Company&apos;s Stock Option Plans (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureStockBasedCompensationTotalStockBasedCompensationRecognizedDetails" id="Role_DisclosureStockBasedCompensationTotalStockBasedCompensationRecognizedDetails">
        <link:definition>100680 - Disclosure - Stock-based Compensation - Total Stock-based Compensation Recognized (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureStockBasedCompensationScheduleOfEstimatedFairValueOfEmployeeStockOptionsAndESPPUsingBlackScholesValuationModelDetails" id="Role_DisclosureStockBasedCompensationScheduleOfEstimatedFairValueOfEmployeeStockOptionsAndESPPUsingBlackScholesValuationModelDetails">
        <link:definition>100690 - Disclosure - Stock-based Compensation - Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetails" id="Role_DisclosureIncomeTaxesAdditionalInformationDetails">
        <link:definition>100700 - Disclosure - Income Taxes - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxesScheduleOfComponentsOfProvisionForBenefitFromIncomeTaxesDetails" id="Role_DisclosureIncomeTaxesScheduleOfComponentsOfProvisionForBenefitFromIncomeTaxesDetails">
        <link:definition>100710 - Disclosure - Income Taxes - Schedule of Components of Provision for (Benefit from) Income Taxes (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxesScheduleOfEffectiveTaxRateReconciliationDetails" id="Role_DisclosureIncomeTaxesScheduleOfEffectiveTaxRateReconciliationDetails">
        <link:definition>100720 - Disclosure - Income Taxes - Schedule of Effective Tax Rate Reconciliation (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxesScheduleOfDeferredIncomeTaxAssetsAndLiabilitiesDetails" id="Role_DisclosureIncomeTaxesScheduleOfDeferredIncomeTaxAssetsAndLiabilitiesDetails">
        <link:definition>100730 - Disclosure - Income Taxes - Schedule of Deferred Income Tax Assets and Liabilities (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureIncomeTaxesScheduleOfReconciliationOfUnrecognizedTaxBenefitsDetails" id="Role_DisclosureIncomeTaxesScheduleOfReconciliationOfUnrecognizedTaxBenefitsDetails">
        <link:definition>100740 - Disclosure - Income Taxes- Schedule of Reconciliation of Unrecognized Tax Benefits (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureDefinedContributionPlanAdditionalInformationDetails" id="Role_DisclosureDefinedContributionPlanAdditionalInformationDetails">
        <link:definition>100750 - Disclosure - Defined Contribution Plan - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://cytomx.com/20201231/taxonomy/role/DisclosureSubsequentEventsAdditionalInformationDetails" id="Role_DisclosureSubsequentEventsAdditionalInformationDetails">
        <link:definition>100760 - Disclosure - Subsequent Events - Additional Information (Details)</link:definition>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:element id="ctmx_CashCashEquivalentsAndShortTermInvestmentsAllowanceForCreditLoss" name="CashCashEquivalentsAndShortTermInvestmentsAllowanceForCreditLoss" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_CashCashEquivalentsAndShortTermInvestmentsAccumulatedGrossUnrealizedGainBeforeTax" name="CashCashEquivalentsAndShortTermInvestmentsAccumulatedGrossUnrealizedGainBeforeTax" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_CashCashEquivalentsAndShortTermInvestmentsAccumulatedGrossUnrealizedLossBeforeTax" name="CashCashEquivalentsAndShortTermInvestmentsAccumulatedGrossUnrealizedLossBeforeTax" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_LesseeOperatingLeaseLiabilityPaymentsDueYearFiveAndThereafter" name="LesseeOperatingLeaseLiabilityPaymentsDueYearFiveAndThereafter" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_DepreciationAndAmortizationExcludingAmortizationOfIntangibleAssets" name="DepreciationAndAmortizationExcludingAmortizationOfIntangibleAssets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_NoncashLeaseExpense" name="NoncashLeaseExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_IncreaseDecreaseInAccruedLiabilitiesIncomeTaxPayableAndOtherLiabilitiesNoncurrent" name="IncreaseDecreaseInAccruedLiabilitiesIncomeTaxPayableAndOtherLiabilitiesNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_AdoptedAndRecentAccountingPronouncementsTextBlock" name="AdoptedAndRecentAccountingPronouncementsTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ResearchAndCollaborationAgreementsTextBlock" name="ResearchAndCollaborationAgreementsTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_LicenseAgreementDisclosureTextBlock" name="LicenseAgreementDisclosureTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_CommonStockTextBlock" name="CommonStockTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ScheduleOfCashCashEquivalentsAndRestrictedCashTableTextBlock" name="ScheduleOfCashCashEquivalentsAndRestrictedCashTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PropertyPlantAndEquipmentEstimatedUsefulLivesTableTextBlock" name="PropertyPlantAndEquipmentEstimatedUsefulLivesTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_SupplementalInformationRelatedToLeasesTableTextBlock" name="SupplementalInformationRelatedToLeasesTableTextBlock" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_MajorCustomerMember" name="MajorCustomerMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AbbVieIrelandUnlimitedCompanyMember" name="AbbVieIrelandUnlimitedCompanyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_BristolMyersSquibbCompanyMember" name="BristolMyersSquibbCompanyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AstellasPharmaIncMember" name="AstellasPharmaIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_NumberOfBusinessActivities" name="NumberOfBusinessActivities" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_SummaryOfSignificantAccountingPoliciesLineItems" name="SummaryOfSignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SummaryOfSignificantAccountingPoliciesTable" name="SummaryOfSignificantAccountingPoliciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ProbodyTechnologyMember" name="ProbodyTechnologyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ComputerEquipmentAndSoftwareMember" name="ComputerEquipmentAndSoftwareMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_OptionsAndESPPToPurchaseCommonStockMember" name="OptionsAndESPPToPurchaseCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_CashCashEquivalentsAndShortTermInvestmentsAmortizedCostBasis" name="CashCashEquivalentsAndShortTermInvestmentsAmortizedCostBasis" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_MoneyMarketFundsIncludedInRestrictedCashMember" name="MoneyMarketFundsIncludedInRestrictedCashMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ComputerEquipmentAndSoftwareGross" name="ComputerEquipmentAndSoftwareGross" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_GoodwillAbstract" name="GoodwillAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_GoodwillAndIntangibleAssetsLineItems" name="GoodwillAndIntangibleAssetsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ScheduleOfGoodwillAndIntangibleAssetsTable" name="ScheduleOfGoodwillAndIntangibleAssetsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AccruedResearchAndClinicalCurrent" name="AccruedResearchAndClinicalCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_AmgenIncMember" name="AmgenIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ImmunoGenIncMember" name="ImmunoGenIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_PfizerIncMember" name="PfizerIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_NumberOfCollaborationAgreements" name="NumberOfCollaborationAgreements" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PercentageOfNetIncomeOrNetLossesRelatedToDevelopmentAndCommercializationCosts" name="PercentageOfNetIncomeOrNetLossesRelatedToDevelopmentAndCommercializationCosts" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_CD71AgreementMember" name="CD71AgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SeattleGeneticsAgreementMember" name="SeattleGeneticsAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_DiscoveryAgreementMember" name="DiscoveryAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_DevelopmentRegulatoryAndCommercialMilestonePaymentsMember" name="DevelopmentRegulatoryAndCommercialMilestonePaymentsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SecondTargetMember" name="SecondTargetMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ContingentPaymentsReceivableUponAchievingDevelopmentRegulatoryAndCommercialMilestones" name="ContingentPaymentsReceivableUponAchievingDevelopmentRegulatoryAndCommercialMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_ContingentPaymentsReceivableProfitSplitOnSales" name="ContingentPaymentsReceivableProfitSplitOnSales" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_MilestonePaymentsReceivedFromRevenueRecognizedDuringPeriod" name="MilestonePaymentsReceivedFromRevenueRecognizedDuringPeriod" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_PaymentsOfSublicenseFees" name="PaymentsOfSublicenseFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_NumberOfTargets" name="NumberOfTargets" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfTargetsSelected" name="NumberOfTargetsSelected" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ContingentMilestonePaymentsReceivable" name="ContingentMilestonePaymentsReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_MilestonePaymentReceived" name="MilestonePaymentReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_AggregateCollaborationTransactionAmount" name="AggregateCollaborationTransactionAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_EstimatedSublicenseFees" name="EstimatedSublicenseFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_NumberOfAccountingUnits" name="NumberOfAccountingUnits" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_MilestonePaymentsReceivable" name="MilestonePaymentsReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_EstimatedResearchServicePeriod" name="EstimatedResearchServicePeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_CommonStockSharesIssuableUnderAgreement" name="CommonStockSharesIssuableUnderAgreement" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PeriodUsedToCalculateWeightedAveragePricePerShare" name="PeriodUsedToCalculateWeightedAveragePricePerShare" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_LockUpPeriodForShareTransactionUnderCollaborationAndLicenseAgreement" name="LockUpPeriodForShareTransactionUnderCollaborationAndLicenseAgreement" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_EstimatedPremiumOnIssuanceOfShares" name="EstimatedPremiumOnIssuanceOfShares" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_CollaborationAndLicenseAgreementMember" name="CollaborationAndLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_EGFRProductsMember" name="EGFRProductsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AmgenProductsMember" name="AmgenProductsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SublicenseAgreementMember" name="SublicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_RegentsOfTheUniversityOfCaliforniaMember" name="RegentsOfTheUniversityOfCaliforniaMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AmgenOtherProductsMember" name="AmgenOtherProductsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ProfitShareUponExerciseOfCoDevelopmentOption" name="ProfitShareUponExerciseOfCoDevelopmentOption" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfAdditionalCollaborationTarget" name="NumberOfAdditionalCollaborationTarget" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ContingentPaymentsPayableUponAchievingDevelopmentRegulatoryAndCommercialMilestones" name="ContingentPaymentsPayableUponAchievingDevelopmentRegulatoryAndCommercialMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_LicensePaymentPercentageEquivalenceToUpfrontAndMilestoneProceeds" name="LicensePaymentPercentageEquivalenceToUpfrontAndMilestoneProceeds" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PremiumOnIssuanceOfSharesAllocatedToCommonStock" name="PremiumOnIssuanceOfSharesAllocatedToCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_EstimatedFairValueOfProducts" name="EstimatedFairValueOfProducts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_RightToExpandNumberOfAdditionalCollaborationTarget" name="RightToExpandNumberOfAdditionalCollaborationTarget" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_AdditionalContingentPaymentsMember" name="AdditionalContingentPaymentsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ResearchAndDevelopmentServiceFees" name="ResearchAndDevelopmentServiceFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_EstimatedResearchServiceTerminationMonthAndYear" name="EstimatedResearchServiceTerminationMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfOncologyTarget" name="NumberOfOncologyTarget" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfCollaborationTarget" name="NumberOfCollaborationTarget" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_CollaborationTargetResearchTerm" name="CollaborationTargetResearchTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PeriodOfNominationOfAdditionalTarget" name="PeriodOfNominationOfAdditionalTarget" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ExtensionOfResearchTermForEachCollaborationTarget" name="ExtensionOfResearchTermForEachCollaborationTarget" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_IncrementsOfExtendedCollaborationTargetResearchTimes" name="IncrementsOfExtendedCollaborationTargetResearchTimes" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_AchievingAdditionalTargetsMember" name="AchievingAdditionalTargetsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_PreClinicalCandidateMember" name="PreClinicalCandidateMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_DrugApplicationForCTLA4Member" name="DrugApplicationForCTLA4Member" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ThirdAndFourthTargetsMember" name="ThirdAndFourthTargetsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AcceleratedRecognitionOfDeferredRevenueMember" name="AcceleratedRecognitionOfDeferredRevenueMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_NumberOfResearchTargetsSelected" name="NumberOfResearchTargetsSelected" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_UpfrontFeeReceived" name="UpfrontFeeReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_EstimatedResearchServiceTerminationDate" name="EstimatedResearchServiceTerminationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ContingentMilestonePaymentsNoLongerEligibleToReceive" name="ContingentMilestonePaymentsNoLongerEligibleToReceive" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_CashPaymentToExecuteAgreement" name="CashPaymentToExecuteAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_MilestonePaymentPayable" name="MilestonePaymentPayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_NumberOfNominatedResearchTargets" name="NumberOfNominatedResearchTargets" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfAdditionalResearchTargetsWithNominationOption" name="NumberOfAdditionalResearchTargetsWithNominationOption" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ContractWithCustomerLiabilityAdditions" name="ContractWithCustomerLiabilityAdditions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_FirstTargetUnderDiscoveryAgreementMember" name="FirstTargetUnderDiscoveryAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SecondTargetUnderDiscoveryAgreementMember" name="SecondTargetUnderDiscoveryAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ContractWithCustomerLiabilityMaturityMonthAndYear" name="ContractWithCustomerLiabilityMaturityMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_RoyaltyObligationsFutureMinimumPaymentsDue" name="RoyaltyObligationsFutureMinimumPaymentsDue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_LicenseTerminationPeriod" name="LicenseTerminationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_UniversityOfCaliforniaSantaBarbaraMember" name="UniversityOfCaliforniaSantaBarbaraMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SublicenseAndMaintenanceFeesMember" name="SublicenseAndMaintenanceFeesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_AbbVieMember" name="AbbVieMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_PaymentOfUpfrontFees" name="PaymentOfUpfrontFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_AnnualLicenseMaintenanceFees" name="AnnualLicenseMaintenanceFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_LicensePaymentTerm" name="LicensePaymentTerm" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PercentageOfRemainingMaintenanceFeesOutstanding" name="PercentageOfRemainingMaintenanceFeesOutstanding" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_FairValueOfCommonStockIssued" name="FairValueOfCommonStockIssued" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_AdditionalSublicenseFees" name="AdditionalSublicenseFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_MilestonePaymentReceivable" name="MilestonePaymentReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_SublicenseFees" name="SublicenseFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_UpfrontLicensePayment" name="UpfrontLicensePayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_LossContingencyAnswerAffirmativeDefensesAndCounterclaimsFilingDate" name="LossContingencyAnswerAffirmativeDefensesAndCounterclaimsFilingDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_LossContingencyAnswerAndCounterclaimsByPlaintiffFilingDate" name="LossContingencyAnswerAndCounterclaimsByPlaintiffFilingDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PatentInfringementLawsuitMember" name="PatentInfringementLawsuitMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SecuritiesClassActionLawsuitMember" name="SecuritiesClassActionLawsuitMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_NumberOfCurrentAndFormerOfficersNamedAsDefendants" name="NumberOfCurrentAndFormerOfficersNamedAsDefendants" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_FirstYearOfLeaseTermMember" name="FirstYearOfLeaseTermMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SecondTwelveMonthsOfLeaseTermMember" name="SecondTwelveMonthsOfLeaseTermMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_TenthYearOfLeaseTermMember" name="TenthYearOfLeaseTermMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_OperatingLeasesRentExpenseAnnualBaseRent" name="OperatingLeasesRentExpenseAnnualBaseRent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_MaximumImprovementAllowance" name="MaximumImprovementAllowance" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_ImprovementAllowanceFromRecoverableRent" name="ImprovementAllowanceFromRecoverableRent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_CashPaidForAmountsIncludedInMeasurementOfLeaseLiabilitiesAbstract" name="CashPaidForAmountsIncludedInMeasurementOfLeaseLiabilitiesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_SupplementalBalanceSheetInformationRelatedToLeasesAbstract" name="SupplementalBalanceSheetInformationRelatedToLeasesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_WeightedAverageRemainingLeaseTermAbstract" name="WeightedAverageRemainingLeaseTermAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_WeightedAverageDiscountRateAbstract" name="WeightedAverageDiscountRateAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_UnderwrittenPublicOfferingMember" name="UnderwrittenPublicOfferingMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_JefferiesMember" name="JefferiesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_PercentageOfSalesCommission" name="PercentageOfSalesCommission" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_SharesIssuedAveragePricePerShare" name="SharesIssuedAveragePricePerShare" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PercentageOfSalesCommissionAndIssuanceCost" name="PercentageOfSalesCommissionAndIssuanceCost" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_StockOptionsIssuedAndOutstanding" name="StockOptionsIssuedAndOutstanding" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForFutureIssuanceUnderEmployeeStockPurchasePlan" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForFutureIssuanceUnderEmployeeStockPurchasePlan" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardVotingRightsOfCommonStockHoldingPercentage" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardVotingRightsOfCommonStockHoldingPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_TwoThousandFifteenStockIncentivePlanMember" name="TwoThousandFifteenStockIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_TwoThousandNineteenStockIncentivePlanMember" name="TwoThousandNineteenStockIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_EarlyExerciseOfEmployeeOptionsMember" name="EarlyExerciseOfEmployeeOptionsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_EmployeeStockPurchasePlanMember" name="EmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_NonEmployeesMember" name="NonEmployeesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAuthorized" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAuthorized" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantGrantsInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantGrantsInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantExercisesInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantExercisesInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantCancelledInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantCancelledInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAbstract" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAuthorized" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAuthorized" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_NetOperatingLossCarrybackPeriod" name="NetOperatingLossCarrybackPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PercentageOfTaxableIncomeFromNetOperatingLossesForSuspensionOfAnnualDeductionLimitation" name="PercentageOfTaxableIncomeFromNetOperatingLossesForSuspensionOfAnnualDeductionLimitation" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_EffectiveIncomeTaxRateReconciliationNetOperatingLossCarryback" name="EffectiveIncomeTaxRateReconciliationNetOperatingLossCarryback" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_EffectiveIncomeTaxRateReconciliationReturnToProvisionAdjustments" name="EffectiveIncomeTaxRateReconciliationReturnToProvisionAdjustments" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_DeferredTaxAssetsLeaseLiability" name="DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_OperatingLossCarryforwardsAmountToExpireIfNotUsed" name="OperatingLossCarryforwardsAmountToExpireIfNotUsed" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_OperatingLossCarryforwardsAmountNotSubjectToExpiration" name="OperatingLossCarryforwardsAmountNotSubjectToExpiration" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_TaxCreditCarryforwardExpirationYear" name="TaxCreditCarryforwardExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_PercentageOfChangeInOwnership" name="PercentageOfChangeInOwnership" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromPriorPeriodTaxPositions" name="UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromPriorPeriodTaxPositions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_OptionPeriodGrantedToUnderwritersToPurchaseAdditionalShares" name="OptionPeriodGrantedToUnderwritersToPurchaseAdditionalShares" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
</xsd:schema>
