-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 aAQxBM2DBQKCRR5RlA7WfA==

<SEC-DOCUMENT>0001179110-10-016279.txt : 20101118
<SEC-HEADER>0001179110-10-016279.hdr.sgml : 20101118
<ACCEPTANCE-DATETIME>20101118154343
ACCESSION NUMBER:		0001179110-10-016279
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20101116
FILED AS OF DATE:		20101118
DATE AS OF CHANGE:		20101118

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			PIEN HOWARD H
		CENTRAL INDEX KEY:			0001187152

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-17999
		FILM NUMBER:		101202769

	MAIL ADDRESS:	
		STREET 1:		C/O MEDAREX, INC
		STREET 2:		707 STATE ROAD
		CITY:			PRINCETON
		STATE:			NJ
		ZIP:			08540

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
</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>2010-11-16</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001187152</rptOwnerCik>
            <rptOwnerName>PIEN HOWARD H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>6 CARRIAGE HOUSE COURT</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHERRY HILL</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>08003</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Share Unit</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-11-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3836</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2011-02-01</value>
                <footnoteId id="F3"/>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3836</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3836</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>7.82</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-11-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6211</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.82</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2011-02-01</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2020-11-16</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6211</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6211</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The deferred share units were issued pursuant to the Issuer's Compensation Policy for Non-Employee Directors and are convertible into Common Stock on a one-to-one basis.</footnote>
        <footnote id="F2">The vested deferred share units are to be settled 100% in shares of Common Stock of the Company upon the reporting person's retirement from the Board of Directors.</footnote>
        <footnote id="F3">The deferred share units vest ratably over a one year period in quarterly increments beginning on February 1, 2011, contingent upon the individual remaining a director as of each vesting date.</footnote>
        <footnote id="F4">Exercisable as to 1553 shares commencing on February 1, 2011, 1553 shares commencing on May 1, 2011, 1552 shares commencing on August 1, 2011 and 1553 shares commencing on November 1, 2011</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Craig Barrows, attorney in fact</signatureName>
        <signatureDate>2010-11-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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