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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2020-11-02T22:47:01.9162515+00:00 -->
<!-- Version            : 5.0.1.513 -->
<!-- Package ID         : 9613b63dd927431aabb61ab156caf9fe -->
<!-- Copyright (c) 2020 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="ctmx_AchievingAdditionalTargetsMember" name="AchievingAdditionalTargetsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_PreClinicalCandidatesMember" name="PreClinicalCandidatesMember" 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_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_CashPaymentToExecuteAgreement" name="CashPaymentToExecuteAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_ImmunoGenIncMember" name="ImmunoGenIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <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_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_SublicenseAndMaintenanceFeesMember" name="SublicenseAndMaintenanceFeesMember" 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_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_PercentageOfSalesCommission" name="PercentageOfSalesCommission" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_JefferiesMember" name="JefferiesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <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_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_NumberOfCurrentAndFormerOfficersNamedAsDefendants" name="NumberOfCurrentAndFormerOfficersNamedAsDefendants" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <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:schema>
