<SEC-DOCUMENT>0001140361-12-039299.txt : 20120904
<SEC-HEADER>0001140361-12-039299.hdr.sgml : 20120903
<ACCEPTANCE-DATETIME>20120904204145
ACCESSION NUMBER:		0001140361-12-039299
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20120831
FILED AS OF DATE:		20120904
DATE AS OF CHANGE:		20120904

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Kelly William J
		CENTRAL INDEX KEY:			0001431114

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-14656
		FILM NUMBER:		121072106

	MAIL ADDRESS:	
		STREET 1:		C/O REPLIGEN CORPORATION
		STREET 2:		41 SEYON STREET, BLDG #1, SUITE 100
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02453

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			REPLIGEN CORP
		CENTRAL INDEX KEY:			0000730272
		STANDARD INDUSTRIAL CLASSIFICATION:	BIOLOGICAL PRODUCTS (NO DIAGNOSTIC SUBSTANCES) [2836]
		IRS NUMBER:				042729386
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		41 SEYON STREET
		STREET 2:		BUILDING 1, SUITE 100
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02453
		BUSINESS PHONE:		7814499560

	MAIL ADDRESS:	
		STREET 1:		41 SEYON STREET
		STREET 2:		BUILDING 1, SUITE 100
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02453
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
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<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-08-31</periodOfReport>

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        <issuerCik>0000730272</issuerCik>
        <issuerName>REPLIGEN CORP</issuerName>
        <issuerTradingSymbol>RGEN</issuerTradingSymbol>
    </issuer>

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            <rptOwnerCik>0001431114</rptOwnerCik>
            <rptOwnerName>Kelly William J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O REPLIGEN CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>41 SEYON STREET, BLDG 1, STE 100</rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02453</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>CFO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
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                <equitySwapInvolved>0</equitySwapInvolved>
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                <transactionShares>
                    <value>80000</value>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <transactionAmounts>
                <transactionShares>
                    <value>10700</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.52</value>
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                <transactionAcquiredDisposedCode>
                    <value>D</value>
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                <sharesOwnedFollowingTransaction>
                    <value>35391</value>
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                <directOrIndirectOwnership>
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        <nonDerivativeTransaction>
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                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionAmounts>
                <transactionShares>
                    <value>9681</value>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
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        <nonDerivativeTransaction>
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                <transactionShares>
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                <transactionPricePerShare>
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    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>4.82</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-08-31</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
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                <transactionFormType>4</transactionFormType>
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            <transactionAmounts>
                <transactionShares>
                    <value>80000</value>
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                <transactionPricePerShare>
                    <value>0</value>
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                <transactionAcquiredDisposedCode>
                    <value>D</value>
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            <exerciseDate>
                <footnoteId id="F1"/>
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            <expirationDate>
                <value>2018-03-31</value>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>80000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>45000</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        <derivativeTransaction>
            <securityTitle>
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            <conversionOrExercisePrice>
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            <transactionDate>
                <value>2012-08-31</value>
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                <transactionShares>
                    <value>10000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
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                    <value>D</value>
                </transactionAcquiredDisposedCode>
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            <exerciseDate>
                <footnoteId id="F2"/>
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            <expirationDate>
                <value>2020-07-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10000</value>
                </underlyingSecurityShares>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>35000</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
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        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">20,000 shares vested and became exercisable at each of the following dates: 3/31/2009, 3/31/2010, 3/31/2011 and 3/31/2012.</footnote>
        <footnote id="F2">5,000 shares each vested and became exercisable at 7/15/2011 and 7/15/2012.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>William J. Kelly (Attorney in Fact)</signatureName>
        <signatureDate>2012-09-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
