

<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2019-05-09T14:56:10.2124777+00:00 -->
<!-- Version            : 3.4.7044.22220 -->
<!-- Package ID         : adbbea97c8a9496dbde3f1fb99048ec3 -->
<!-- Copyright (c) 2019 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="ctmx_ThirdAndFourthTargetsMember" name="ThirdAndFourthTargetsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ContingentMilestonePaymentsReceivableTerminated" name="ContingentMilestonePaymentsReceivableTerminated" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_NumberOfResearchTargetsSelected" name="NumberOfResearchTargetsSelected" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfSalesIndicatorsForEachResearchTargetProgram" name="NumberOfSalesIndicatorsForEachResearchTargetProgram" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="ctmx_NumberOfNonOncologyTarget" name="NumberOfNonOncologyTarget" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <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_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_LicenseReceivablesMember" name="LicenseReceivablesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_ResearchServicesJointResearchCommitteeParticipationAndTechnologyImprovementsMember" name="ResearchServicesJointResearchCommitteeParticipationAndTechnologyImprovementsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="ctmx_MilestonesPaymentsMadeDuringPeriod" name="MilestonesPaymentsMadeDuringPeriod" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_ContingentPaymentsReceivableUponAchievingDevelopmentAndRegulatoryMilestones" name="ContingentPaymentsReceivableUponAchievingDevelopmentAndRegulatoryMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_ContingentPaymentsReceivableUponAchievingSalesMilestones" name="ContingentPaymentsReceivableUponAchievingSalesMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="ctmx_ContingentPaymentsPayableUponAchievingDevelopmentAndRegulatoryMilestones" name="ContingentPaymentsPayableUponAchievingDevelopmentAndRegulatoryMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_ContingentPaymentsPayableUponAchievingSalesMilestones" name="ContingentPaymentsPayableUponAchievingSalesMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_MilestonePaymentAccrued" name="MilestonePaymentAccrued" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="ctmx_EstimatedFairValueOfAssetsAndServices" name="EstimatedFairValueOfAssetsAndServices" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <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_AmortizationPeriodOfDeferredRevenue" name="AmortizationPeriodOfDeferredRevenue" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="ctmx_ContractWithCustomerLiabilityMaturityDate" name="ContractWithCustomerLiabilityMaturityDate" type="xbrli:dateItemType" 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_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_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_PaymentOfUpfrontFees" name="PaymentOfUpfrontFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
</xsd:schema>





