<SUBMISSION>
<ACCESSION-NUMBER>0001179110-03-012373
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20031209
<FILING-DATE>20031211
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>ERVIN HOWARD G
<CIK>0001193211
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>000-21937
<FILM-NUMBER>031050038
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>C/O CERUS CORP
<STREET2>2411 STENWELL DRIVE
<CITY>CONCORD
<STATE>CA
<ZIP>94520
</MAIL-ADDRESS>
</REPORTING-OWNER>
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>CERUS CORP
<CIK>0001020214
<ASSIGNED-SIC>2836
<IRS-NUMBER>680262011
<STATE-OF-INCORPORATION>DE
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>2411 STANWELL DR
<CITY>CONCORD
<STATE>CA
<ZIP>94520
<PHONE>9252886000
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>2525 STANWELL DRIVE
<STREET2>STE 300
<CITY>CONCORD
<STATE>CA
<ZIP>94520
</MAIL-ADDRESS>
<FORMER-COMPANY>
<FORMER-CONFORMED-NAME>CERUS TECHNOLOGIES INC
<DATE-CHANGED>19960731
</FORMER-COMPANY>
</ISSUER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-12-09</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0001020214</issuerCik>
        <issuerName>CERUS CORP</issuerName>
        <issuerTradingSymbol>CERS</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001193211</rptOwnerCik>
            <rptOwnerName>ERVIN HOWARD G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CERUS CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>2411 STANWELL DRIVE</rptOwnerStreet2>
            <rptOwnerCity>CONCORD</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94520</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>Vice President, Legal Affairs</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable></nonDerivativeTable>
<derivativeTable><derivativeTransaction><securityTitle><value>Option (Right to Buy)</value>
</securityTitle>
<conversionOrExercisePrice><value>4.25</value>
</conversionOrExercisePrice>
<transactionDate><value>2003-12-09</value>
</transactionDate>
<deemedExecutionDate><value>2003-12-09</value>
</deemedExecutionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>A</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>24600</value>
</transactionShares>
<transactionPricePerShare><footnoteId id="F2"></footnoteId>
</transactionPricePerShare>
<transactionAcquiredDisposedCode><value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<exerciseDate><footnoteId id="F1"></footnoteId>
</exerciseDate>
<expirationDate><value>2013-12-08</value>
</expirationDate>
<underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares><value>24600</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>24600</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</derivativeTransaction>
</derivativeTable>
<footnotes><footnote id="F1">Option vests monthly over 48 months from November 14, 2003.</footnote>
<footnote id="F2">Not applicable.</footnote>
</footnotes>
<ownerSignature><signatureName>Howard G. Ervin by Lori Roll, attorney-in-fact</signatureName>
<signatureDate>2003-12-11</signatureDate>
</ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
