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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 OxYfTJpZ+Mm80ADFL/3FWbgnVMfUso3ujN97q32eW2l8lG3X7DAVKOHcXq2mTlxp
 YmSDKRqLfLhDErT7llBSzw==

<SEC-DOCUMENT>0001179110-10-011871.txt : 20100726
<SEC-HEADER>0001179110-10-011871.hdr.sgml : 20100726
<ACCEPTANCE-DATETIME>20100726140452
ACCESSION NUMBER:		0001179110-10-011871
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100723
FILED AS OF DATE:		20100726
DATE AS OF CHANGE:		20100726

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			IMMUNOGEN INC
		CENTRAL INDEX KEY:			0000855654
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				042726691
		STATE OF INCORPORATION:			MA
		FISCAL YEAR END:			0630

	BUSINESS ADDRESS:	
		STREET 1:		830 WINTER ST
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		(781)895-0600

	MAIL ADDRESS:	
		STREET 1:		830 WINTER ST
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			LAMBERT JOHN
		CENTRAL INDEX KEY:			0001198338

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-17999
		FILM NUMBER:		10969122

	MAIL ADDRESS:	
		STREET 1:		C/O IMMUNOGEN
		STREET 2:		128 SIDNEY ST.
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-07-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000855654</issuerCik>
        <issuerName>IMMUNOGEN INC</issuerName>
        <issuerTradingSymbol>IMGN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001198338</rptOwnerCik>
            <rptOwnerName>LAMBERT JOHN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O IMMUNOGEN, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>830 WINTER STREET</rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Executive Vice President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>9.14</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-07-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>80000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>9.14</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2011-07-23</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2020-07-23</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>80000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>80000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Exercisable as to 26,667 shares commencing on July 23, 2011, 26,666 shares commencing on July 23, 2012 and 26,667 shares commencing on July 23, 2013.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Craig Barrows, attorney in fact</signatureName>
        <signatureDate>2010-07-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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