Exhibit 10.2

EXECUTION COPY

REGISTRATION RIGHTS AGREEMENT

This Registration Rights Agreement (this “Agreement”) is made and entered into as of June 20, 2008, by and between ImmunoGen, Inc., a Massachusetts corporation (the “Company”), and Ziff Asset Management, L.P., a Delaware limited partnership (the “Purchaser”).

This Agreement is made pursuant to the Securities Purchase Agreement, dated as of June 20, 2008, between the Company and the Purchaser (the “Purchase Agreement”).

NOW, THEREFORE, IN CONSIDERATION of the mutual covenants contained in this Agreement, and for other good and valuable consideration the receipt and adequacy of which are hereby acknowledged, the Company and the Purchaser agree as follows:

1. Definitions. Capitalized terms used and not otherwise defined herein that are defined in the Purchase Agreement shall have the meanings given such terms in the Purchase Agreement. As used in this Agreement, the following terms shall have the respective meanings set forth in this Section 1:

“Commission” means the United States Securities and Exchange Commission or any other federal agency at the time administering the Securities Act.

“Common Stock” means the common stock, par value $0.01 per share, of the Company.

“Effective Date” means the date that the Registration Statement filed pursuant to Section 2(a) is first declared effective by the Commission.

“Effectiveness Date” means: (a) with respect to the Initial Registration Statement, the 90th day following the Closing, (b) with respect to any additional Registration Statements that may be required pursuant to Section 2 hereof, the 90th day following the date on which the Company first knows, or reasonably should have known, that such additional Registration Statement is required under such Section.

“Effectiveness Period” shall have the meaning set forth in Section 2(a).

“Event” shall have the meaning set forth in Section 2(c).

“Event Date” shall have the meaning set forth in Section 2(c).

“Exchange Act” means the Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder.

“Filing Date” means: (a) with respect to the initial Registration Statement required to be filed to cover the resale by the Holders of the Registrable Securities, the 45th day following the Closing, and (b) with respect to any additional Registration Statements that may be required pursuant to Section 2 hereof, the 45th day following the date on which the Company first knows,

or reasonably should have known, that such additional Registration Statement is required under such Section.

“Holder” or “Holders” means the holder or holders, as the case may be, from time to time of Registrable Securities.

“Indemnified Party” shall have the meaning set forth in Section 5(c)(i).

“Indemnifying Party” shall have the meaning set forth in Section 5(c)(i).

“Initial Registration Statement” shall mean the initial Registration Statement required to be filed to cover the resale by the Holders of the Registrable Securities pursuant to Section 2(a).

“Inspectors” shall have the meaning set forth in Section 3(l).

“Losses” shall have the meaning set forth in Section 5(a).

“Person” means an individual or corporation, partnership, trust, incorporated or unincorporated association, joint venture, limited liability company, joint stock company, government (or an agency or subdivision thereof) or other entity of any kind.

“Proceeding” means an action, claim, suit, investigation or proceeding (including, without limitation, an investigation or partial proceeding, such as a deposition), whether commenced or threatened.

“Prospectus” means the prospectus included in a Registration Statement (including, without limitation, a prospectus that includes any information previously omitted from a prospectus filed as part of an effective registration statement in reliance upon Rule 430A or Rule 430B promulgated under the Securities Act), as amended or supplemented by any prospectus supplement, with respect to the terms of the offering of any portion of the Registrable Securities covered by a Registration Statement, and all other amendments and supplements to the Prospectus, including post-effective amendments, and all material incorporated by reference or deemed to be incorporated by reference in such Prospectus.

“Reduction Securities” shall have the meaning set forth in Section 2(d).

“Registrable Securities” means (i) the Shares issued pursuant to the Purchase Agreement and (ii) any other shares of Common Stock issued as (or issuable upon conversion or exercise of any warrant, right or other security which is issued as) a dividend or other distribution with respect to, in exchange for or in replacement of the Shares; provided, however, that Shares which are Registrable Securities shall cease to be Registrable Securities upon the earlier of (x) any sale of such Registrable Securities pursuant to a Registration Statement or Rule 144, or (y) following any sale of such Registrable Securities by the Purchaser (other than pursuant to a Registration Statement or Rule 144) to any Person other than an Affiliate of the Purchaser who, after giving effect to such sale, owns less than 10% of the outstanding shares of Common Stock, at such time as such Person is able to sell all of his or its Registrable Securities

pursuant to Rule 144(b)(1).

“Registration Statement” means each of the following: (i) the Initial Registration Statement, and (ii) each additional registration statement, if any, contemplated by Section 2, and including, in each case, the Prospectus, amendments and supplements to each such registration statement or Prospectus, including pre- and post-effective amendments, all exhibits thereto, and all material incorporated by reference or deemed to be incorporated by reference in such registration statement.

“Rule 144” means Rule 144 promulgated by the Commission pursuant to the Securities Act, as such Rule may be amended from time to time, or any similar rule or regulation hereafter adopted by the Commission having substantially the same effect as such Rule.

“Rule 415” means Rule 415 promulgated by the Commission pursuant to the Securities Act, as such Rule may be amended from time to time, or any similar rule or regulation hereafter adopted by the Commission having substantially the same effect as such Rule.

“Rule 424” means Rule 424 promulgated by the Commission pursuant to the Securities Act, as such Rule may be amended from time to time, or any similar rule or regulation hereafter adopted by the Commission having substantially the same effect as such Rule.

“Securities Act” means the Securities Act of 1933, as amended, and the rules and regulations promulgated thereunder.

“Shares” shall have the meaning set forth in the Purchase Agreement.

“Trading Day” shall have the meaning set forth in the Purchase Agreement.

2. Registration.

(a) On or prior to each Filing Date, the Company shall prepare and file with the Commission a Registration Statement covering the resale of all Registrable Securities not already covered by an existing and effective Registration Statement (except as provided in Section 2(b) and Section 2(d)) for an offering to be made on a continuous basis pursuant to Rule 415. The Registration Statement shall be on Form S-3 (except if the Company is not then eligible to register for resale the Registrable Securities on Form S-3, in which case such registration shall be on another appropriate form for such purpose reasonably acceptable to the Purchaser) and shall contain (except if otherwise required pursuant to written comments received from the Commission upon a review of such Registration Statement) the “Plan of Distribution” in substantially the form attached hereto as Annex A, with such changes as may be requested by the Holders. The Company shall use its commercially reasonable efforts to cause each Registration Statement to be declared effective under the Securities Act as soon as possible after the respective dates of filing thereof, but, in any event, no later than the Effectiveness Date for such Registration Statement, and shall use its commercially reasonable efforts to keep the Registration Statement continuously effective under the Securities Act until the date on which all Registrable Securities covered by such Registration Statement have ceased to be Registrable Securities (the “Effectiveness Period”), subject to Section 6(d) hereof. It is agreed and understood that the

Company shall, from time to time, be obligated to file one or more additional Registration Statements to cover any Registrable Securities which are not registered for resale pursuant to a pre-existing Registration Statement.

(b) Notwithstanding anything contained herein to the contrary, including the fact that such Registrable Securities may be registered pursuant to the Registration Statement referred to in Section 2(d) below, in the event that the Commission limits the amount of Registrable Securities that may be included and sold by Holders in any Registration Statement, including the Initial Registration Statement, pursuant to Rule 415 or any other basis, the Company may reduce the number of Registrable Securities included in such Registration Statement on behalf of the Holders (in proportion to the respective numbers of Registrable Securities represented by Shares held by such Holders ). In such event the Company shall give the Holders prompt notice of the number of the Registrable Securities excluded and the Company will not be liable for any liquidated damages under Section 2(c), or otherwise under this Agreement, in connection with the excluded Registrable Securities. The Company shall use its commercially reasonable efforts at the first opportunity that is permitted by the Commission to register for resale the Registrable Securities that were excluded from being registered on such Registration Statement. Such new Registration Statement shall be on Form S-3 (except if the Company is not then eligible to register for resale the Registrable Securities on Form S-3, in which case such registration shall be on another appropriate form for such purpose reasonably acceptable to the Purchaser) and shall contain (except if otherwise required pursuant to written comments received from the Commission upon a review of such Registration Statement) the “Plan of Distribution” in substantially the form attached hereto as Annex A, with such changes as the Holders may request. The Company shall use its commercially reasonable efforts to cause each such Registration Statement to be declared effective under the Securities Act as soon as possible after the respective dates of filing thereof, but, in any event, no later than the Effectiveness Date for such Registration Statement, and shall use its commercially reasonable efforts to keep such Registration Statement continuously effective under the Securities Act during the entire Effectiveness Period, subject to Section 6(d) hereof.

(c) Except as provided in Section 2(b), if: (i) a Registration Statement is not filed on or prior to its Filing Date, (ii) a Registration Statement is not declared effective by the Commission on or prior to its required Effectiveness Date, or (iii) after its Effective Date, such Registration Statement ceases for any reason to be effective and available to the Holders as to all Registrable Securities to which it is required to cover at any time prior to the expiration of its Effectiveness Period for an aggregate of more than 20 consecutive Trading Days or an aggregate of 50 Trading Days (which need not be consecutive) in any given 360-day period, (any such failure or breach being referred to as an “Event,” and for purposes of clauses (i) or (ii) the date on which such Event occurs, and for purposes of clause (iii) the date on which such 20 consecutive or 50 Trading Day-period (as applicable) is exceeded, being referred to as the “Event Date”), then, in addition to any other rights available to the Holders: (x) on such Event Date the Company shall pay to each Holder an amount in cash, as liquidated damages and not as a penalty, equal to 1% of the aggregate purchase price paid by such Holder pursuant to the Purchase Agreement for its Registrable Securities then held; and (y) on each monthly anniversary of each such Event Date (if the applicable Event shall not have been cured by such date) until the applicable Event is cured, the Company shall pay to each Holder an amount in

cash, as partial liquidated damages and not as a penalty, equal to 1% of the aggregate purchase price paid by such Holder pursuant to the Purchase Agreement for its Registrable Securities then held; provided, that all periods shall be tolled, with respect to a Holder, by the number of Trading Days during which such Holder fails to provide the Company with information regarding such Holder which was reasonably requested by the Company in order to effect the registration of such Holder’s Registrable Securities pursuant to Section 6(e) hereof. The partial liquidated damages pursuant to the terms hereof shall apply on a pro rata basis for any portion of a month prior to the cure of an Event.

(d) In the event that the number of Registrable Securities included in the Initial Registration Statement or any subsequent Registration Statement are reduced as provided in Section 2(b) above (such Registrable Securities, the “Reduction Securities”), the Company shall prepare and file with the Commission a Registration Statement covering the resale of the Reduction Securities on Form S-1 (or another appropriate form for such purpose reasonably acceptable to the Purchaser). The Registration Statement shall contain (except if otherwise required pursuant to written comments received from the Commission upon a review of such Registration Statement) the “Plan of Distribution” in substantially the form attached hereto as Annex A, with such changes as the Holders may request. The Company shall use its commercially reasonable efforts to cause such Registration Statement to be declared effective under the Securities Act as soon as possible after the date of filing thereof, but, in any event, no later than the Effectiveness Date for such Registration Statement, and shall use its commercially reasonable efforts to keep the Registration Statement continuously effective under the Securities Act for the Effectiveness Period, subject to Section 6(d) hereof. In the event that any Reduction Securities become registered pursuant to a Registration Statement on Form S-3 pursuant to Section 2(b) or otherwise, such Reduction Securities may be removed from the Registration Statement on Form S-1 contemplated by this Section 2(d).

3. Registration Procedures. In connection with the Company’s registration obligations hereunder, the Company shall:

(a) Not less than five Trading Days prior to the filing of a Registration Statement or any related Prospectus or any amendment or supplement thereto, the Company shall furnish to the Holders copies of all such documents proposed to be filed (other than those incorporated by reference). In the event that any Registration Statement is on Form S-1 (or other form which does not permit incorporation by reference), the Company shall not be required to furnish to the Holders any prospectus supplement containing information included in a report or proxy statement filed under the Exchange Act that would be incorporated by reference in such Registration Statement if such Registration Statement were on Form S-3 (or other form which permits incorporation by reference). The Company shall duly consider any comments made by Holders and received by the Company not later than two Trading Days prior to the filing of the Registration Statement, and shall not file any Registration Statement or any related Prospectus or any amendment or supplement thereto in a form to which such Holders reasonably object.

(b) (i) Prepare and file with the Commission such amendments, including post-effective amendments, to each Registration Statement and the Prospectus used in connection therewith as may be necessary to keep such Registration Statement continuously effective as to

the applicable Registrable Securities for its Effectiveness Period and prepare and file with the Commission such additional Registration Statements in order to register for resale under the Securities Act all of the Registrable Securities; (ii) cause the related Prospectus to be amended or supplemented by any required Prospectus supplement, and as so supplemented or amended to be filed pursuant to Rule 424; (iii) respond as promptly as reasonably possible to any comments received from the Commission with respect to each Registration Statement or any amendment thereto and, as promptly as reasonably possible provide the Holders true and complete copies of all correspondence from and to the Commission relating to such Registration Statement, excluding any comments that would result in the disclosure to the Holders of material and non-public information concerning the Company; and (iv) comply in all material respects with the provisions of the Securities Act and the Exchange Act with respect to the Registration Statements and the disposition of all Registrable Securities covered by each Registration Statement.

(c) Notify the Holders as promptly as reasonably possible (and, in the case of clause (i)(A) below, not less than five Trading Days prior to such filing) and confirm such notice in writing no later than one Trading Day following the day: (i)(A) when a Prospectus or any prospectus supplement (but only to the extent notice is required under Section 3(a) above) or post-effective amendment to a Registration Statement is proposed to be filed; (B) when the Commission notifies the Company whether there will be a “review” of such Registration Statement and whenever the Commission comments in writing on such Registration Statement (in which case the Company shall provide true and complete copies thereof and all written responses thereto to each of the Holders, excluding information that the Company believes would constitute material and non-public information regarding the Company); and (C) with respect to each Registration Statement or any post-effective amendment, when the same has been declared effective; (ii) of any request by the Commission or any other Federal or state governmental authority for amendments or supplements to a Registration Statement or Prospectus or for additional information that pertains to the Holders as Selling Stockholders or the Plan of Distribution; (iii) of the issuance by the Commission of any stop order suspending the effectiveness of a Registration Statement covering any or all of the Registrable Securities or the initiation of any Proceedings for that purpose; (iv) of the receipt by the Company of any notification with respect to the suspension of the qualification or exemption from qualification of any of the Registrable Securities for sale in any jurisdiction, or the initiation or threatening of any Proceeding for such purpose; (v) of the occurrence of any event or passage of time that makes the financial statements included or incorporated by reference in a Registration Statement ineligible for inclusion or incorporation by reference therein or any statement made in such Registration Statement or Prospectus or any document incorporated or deemed to be incorporated therein by reference untrue in any material respect or that requires any revisions to such Registration Statement, Prospectus or other documents so that, in the case of such Registration Statement or the Prospectus, as the case may be, it will not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein, in light of the circumstances under which they were made, not misleading; and (vi) of the occurrence or existence of any pending corporate development with respect to the Company that the Company believes may be material and that, in the determination of the Company, makes it not in the best interest of the Company to allow continued availability of a Registration Statement or Prospectus; provided, that any and all of

such information shall remain confidential to each Holder until such information otherwise becomes public, unless disclosure by a Holder is required by law; provided, further, that notwithstanding each Holder’s agreement to keep such information confidential, each such Holder makes no acknowledgement that any such information is material, non-public information.

(d) Use its commercially reasonable efforts to prevent the issuance of, or, if issued, obtain the withdrawal of (i) any order suspending the effectiveness of a Registration Statement, or (ii) any suspension of the qualification (or exemption from qualification) of any of the Registrable Securities for sale in any jurisdiction, at the earliest practicable moment and to notify each Holder who holds Registrable Securities being sold of the issuance of any such order and the resolution thereof or its receipt of actual notice of the initiation or threat of any proceeding for such purpose.

(e) Furnish to each Holder, without charge, at least one conformed copy of each Registration Statement and each amendment thereto and all exhibits to the extent reasonably requested by such Person (including those previously furnished or incorporated by reference) promptly after the filing of such documents with the Commission.

(f) Promptly deliver to each Holder, without charge, (i) as many copies of each Prospectus or Prospectuses (including each form of prospectus) and each amendment or supplement thereto as such Persons may reasonably request and (ii) such other documents, including copies of any preliminary or final prospectus, as such Holder may reasonably request from time to time in order to facilitate the disposition of the Registrable Securities owned by such Holder. Subject to Section 6(d) hereof, the Company hereby consents to the use of such Prospectus and each amendment or supplement thereto by each of the selling Holders in connection with the offering and sale of the Registrable Securities covered by such Prospectus and any amendment or supplement thereto.

(g) Prior to any public offering of Registrable Securities, use its commercially reasonable efforts to (i) register or qualify or cooperate with the selling Holders in connection with the registration or qualification (or exemption from such registration or qualification) of such Registrable Securities for offer and sale under the securities or Blue Sky laws of those jurisdictions within the United States as any Holder reasonably requests in writing to keep each such registration or qualification (or exemption therefrom) effective during the Effectiveness Period, (ii) prepare and file in those jurisdictions such amendments (including post-effective amendments) and supplements to such registrations and qualifications as may be necessary to maintain the effectiveness thereof during the Effectiveness Period, (iii) take such other actions as may be reasonably necessary to maintain such registrations and qualifications in effect at all times during the Effectiveness Period and (iv) to do any and all other acts or things necessary or advisable to enable the disposition in such jurisdictions of the Registrable Securities covered by the Registration Statements; provided, that the Company shall not be required to qualify generally to do business in any jurisdiction where it is not then qualified, or subject the Company to any material tax in any such jurisdiction where it is not then so subject, or file a general consent to service of process in any such jurisdiction.

(h) If requested by the Holders, cooperate with the Holders to facilitate the timely preparation and delivery of certificates representing Registrable Securities to be delivered to a transferee pursuant to the Registration Statements, which certificates shall be free of all restrictive legends, and to enable such Registrable Securities to be in such denominations and registered in such names as any such Holders may request.

(i) Upon the occurrence of any event contemplated by Section 3(c)(vi), as promptly as reasonably possible, prepare a supplement or amendment, including a post-effective amendment, to the affected Registration Statements or a supplement to the related Prospectus or any document incorporated or deemed to be incorporated therein by reference, and file any other required document so that, as thereafter delivered, no Registration Statement nor any Prospectus will contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein, in light of the circumstances under which they were made, not misleading. If the Company notifies the Holders in accordance with Section 3(c)(vi) to suspend the use of any Prospectus until the requisite changes to such Prospectus have been made, then the Holders shall suspend use of such Prospectus. The Company will use its commercially reasonable efforts to ensure that the use of the Prospectus may be resumed as promptly as is practicable. The Company shall be entitled to exercise its right to suspend the availability of a Registration Statement and Prospectus for a period not to exceed sixty (60) calendar days (which need not be consecutive days) in any 12-month period.

(j) The Company may require each selling Holder to furnish to the Company a certified statement as to the number of shares of Common Stock beneficially owned by such Holder and any Affiliate thereof, and, if required by the Commission, the natural persons thereof that have voting and dispositive control over the shares.

(k) If any Holder reasonably requests, the Company shall use its commercially reasonable efforts to obtain an opinion from the Company’s counsel and a “comfort” letter from the Company’s independent public accountants in customary form and covering such matters as are customarily covered by such opinions and “comfort” letters delivered to underwriters in underwritten public offerings, which opinion and letter shall be reasonably satisfactory to the underwriter, if any, and furnish, on the date of the effectiveness of the Registration Statement and thereafter from time to time on such dates as a Holder may reasonably request, to each Holder participating in the offering and to each underwriter, if any, a copy of such opinion and letter addressed to such Holder or underwriter.

(l) If any Holder reasonably requests, the Company shall promptly make available for inspection by (i) such Holder, (ii) its legal counsel and (iii) one firm of accountants or other agents retained by the Holders (collectively, the “Inspectors”), all pertinent financial and other records, and pertinent corporate documents and properties of the Company, as shall be reasonably deemed necessary by each Inspector, and cause the Company’s officers, directors and employees to supply all information which any Inspector may reasonably request; provided, however, that if any such information is designated by the Company as being confidential, upon request by the Company, such Holder shall enter into a customary confidentiality agreement with respect to such information. Each Holder agrees that any non-public information obtained by it as a result of such inspections shall be deemed confidential and acknowledges its obligations

under the federal securities laws not to trade any securities of the Company on the basis of material non-public information. In addition, the Company shall not be obligated under this section to disclose any information that is subject to a confidentiality agreement between the Company and any third party, provided that the Company shall use its commercially reasonable efforts to seek the appropriate third party consent to such disclosure to the Inspectors and, in connection with an underwritten offering, the Company shall provide any such information to the underwriter of such offering and its legal counsel, pursuant to customary due diligence procedures for underwritten securities offerings. Nothing herein (or in any other confidentiality agreement between the Company and any Holder) shall be deemed to limit the Holders’ ability to sell Registrable Securities in a manner which is otherwise consistent with applicable laws and regulations.

(m) The Company shall hold in confidence and not make any disclosure of information concerning a Holder provided to the Company unless (i) disclosure of such information is necessary to comply with federal or state securities laws, (ii) the disclosure of such information is necessary to avoid or correct a misstatement or omission in any Registration Statement, (iii) the release of such information is ordered pursuant to a subpoena or other final, non-appealable order from a court or governmental body of competent jurisdiction, or (iv) such information has been made generally available to the public other than by disclosure in violation of this Agreement or any other agreement. The Company agrees that it shall, upon learning that disclosure of such information concerning a Holder is sought in or by a court or governmental body of competent jurisdiction or through other means, give prompt written notice to such Holder and allow such Holder, at the Holder’s expense, to undertake appropriate action to prevent disclosure of, or to obtain a protective order for, such information.

(n) Enter into such customary agreements (including, if applicable, an underwriting agreement) and take such other actions as the Holders participating in such offering shall reasonably request in order to expedite or facilitate the disposition of their Registrable Securities (it being understood that the Holders of the Registrable Securities which are to be distributed by any underwriters shall be parties to any such underwriting agreement and may, at their option, require that the Company make to and for the benefit of such Holders the representations, warranties and covenants and indemnifications of the Company which are being made to and for the benefit of such underwriters).

4. Registration Expenses. All fees and expenses incident to the Company’s performance of or compliance with its obligations under this Agreement (excluding any underwriting discounts and selling commissions) shall be borne by the Company whether or not any Registrable Securities are sold pursuant to a Registration Statement. The fees and expenses referred to in the foregoing sentence shall include, without limitation, (i) all registration and filing fees (including, without limitation, fees and expenses (A) with respect to filings required to be made with the Trading Market on which the Common Stock is then listed for trading, and (B) in compliance with applicable state securities or Blue Sky laws), (ii) printing expenses (including, without limitation, expenses of printing certificates for Registrable Securities and of printing prospectuses if the printing of prospectuses is reasonably requested by the holders of a majority of the Registrable Securities included in the Registration Statement), (iii) messenger, telephone and delivery expenses, (iv) reasonable fees and disbursements of counsel for the

Company, (v) Securities Act liability insurance, if the Company so desires such insurance, (vi) fees and disbursements of all independent certified public accountants (including, without limitation, the expenses of any “comfort” letters), and (vii) reasonable fees and expenses of all other Persons retained by the Company in connection with the consummation of the transactions contemplated by this Agreement. In addition, the Company shall be responsible for all of its internal expenses incurred in connection with the consummation of the transactions contemplated by this Agreement (including, without limitation, all salaries and expenses of its officers and employees performing legal or accounting duties), the expense of any annual audit and the fees and expenses incurred in connection with the listing of the Registrable Securities on any securities exchange as required hereunder. The Company shall reimburse the Holders for the fees and disbursements of one legal counsel representing the Holders in connection with registration, filing or qualification pursuant to Sections 2 and 3 of this Agreement which amount shall be limited to $25,000.

5. Indemnification.

(a) Indemnification by the Company. The Company shall, notwithstanding any termination of this Agreement, indemnify and hold harmless each Holder, the officers, directors, agents, partners, members, stockholders and employees of each Holder, each Person who controls any such Holder (within the meaning of Section 15 of the Securities Act or Section 20 of the Exchange Act) and the officers, directors, agents, partners, members, stockholders and employees of each such controlling Person, to the fullest extent permitted by applicable law, from and against any and all losses, claims, damages, liabilities, costs (including, without limitation, reasonable costs of preparation and reasonable attorneys’ fees) and expenses (collectively, “Losses”), as incurred, arising out of or relating to (i) any untrue or alleged untrue statement of a material fact contained in any Registration Statement, any Prospectus or any form of prospectus or in any amendment or supplement thereto, (ii) any omission or alleged omission of a material fact required to be stated therein or necessary to make the statements therein (in the case of any Prospectus or form of prospectus or supplement thereto, in light of the circumstances under which they were made) not misleading, (iii) any violation or alleged violation by the Company of the Securities Act, the Exchange Act, any other law, including, without limitation, any state securities law, or any rule or regulation thereunder relating to the offer or sale of the Registrable Securities pursuant to a Registration Statement and (iv) any violation of this Agreement, except to the extent, but only to the extent, that, with respect to clauses (i) and (ii), such untrue statements, alleged untrue statements, omissions or alleged omissions are based solely upon information regarding such Holder furnished in writing to the Company by such Holder expressly for use therein. The Company shall notify the Holders promptly of the institution, threat or assertion of any Proceeding of which the Company is aware in connection with the transactions contemplated by this Agreement.

(b) Indemnification by Holders. Each Holder shall, notwithstanding any termination of this Agreement, severally and not jointly, indemnify and hold harmless the Company, its directors, officers, agents and employees, each Person who controls the Company (within the meaning of Section 15 of the Securities Act and Section 20 of the Exchange Act), and the directors, officers, agents, partners, members, stockholders or employees of such controlling Persons, to the fullest extent permitted by applicable law, from and against all Losses, as

incurred, arising solely out of or based solely upon any untrue statement of a material fact contained in any Registration Statement, any Prospectus, or any form of prospectus, or in any amendment or supplement thereto, or arising solely out of or based solely upon any omission of a material fact required to be stated therein or necessary to make the statements therein (in the case of any Prospectus, or any form of prospectus or supplement thereto, in light of the circumstances under which they were made) not misleading to the extent, but only to the extent that, such untrue statements or omissions are based solely upon information regarding such Holder furnished in writing to the Company by such Holder expressly for use therein. In no event shall the liability of any selling Holder hereunder be greater in amount than the dollar amount of the net proceeds received by such Holder upon the sale of the Registrable Securities giving rise to such indemnification obligation.

(c) Conduct of Indemnification Proceedings.

(i) If any Proceeding shall be brought or asserted against any Person entitled to indemnity hereunder (an “Indemnified Party”), such Indemnified Party shall promptly notify the Person from whom indemnity is sought (the “Indemnifying Party”) in writing, and the Indemnifying Party shall assume the defense thereof, including the employment of counsel reasonably satisfactory to the Indemnified Party and the payment of all fees and expenses incurred in connection with defense thereof; provided, that the failure of any Indemnified Party to give such notice shall not relieve the Indemnifying Party of its obligations or liabilities pursuant to this Agreement, except (and only) to the extent that it shall be finally determined by a court of competent jurisdiction (which determination is not subject to appeal or further review) that such failure shall have proximately and materially adversely prejudiced the Indemnifying Party.

(ii) An Indemnified Party shall have the right to employ separate counsel in any such Proceeding and to participate in the defense thereof, but the fees and expenses of such counsel shall be at the expense of such Indemnified Party or Parties unless: (1) the Indemnifying Party has agreed in writing to pay such fees and expenses; (2) the Indemnifying Party shall have failed promptly to assume the defense of such Proceeding and to employ counsel reasonably satisfactory to such Indemnified Party in any such Proceeding; or (3) the named parties to any such Proceeding (including any impleaded parties) include both such Indemnified Party and the Indemnifying Party, and such Indemnified Party shall have been advised by counsel that a conflict of interest is likely to exist if the same counsel were to represent such Indemnified Party and the Indemnifying Party (in which case, if such Indemnified Party notifies the Indemnifying Party in writing that it elects to employ separate counsel at the expense of the Indemnifying Party, the Indemnifying Party shall not have the right to assume the defense thereof and such counsel shall be at the expense of the Indemnifying Party); provided, that the Indemnifying Party shall not be liable for the fees and expenses of more than one separate firm of attorneys at any time for all Indemnified Parties pursuant to this Section 5(c). The Indemnifying Party shall not be liable for any settlement of any such Proceeding effected without its written consent, which consent shall not be unreasonably withheld. No Indemnifying Party shall, without the prior written consent of the Indemnified Party, effect any settlement of any pending Proceeding in respect of which

any Indemnified Party is a party, unless such settlement includes an unconditional release of such Indemnified Party from all liability on claims that are the subject matter of such Proceeding.

(iii) All fees and expenses of the Indemnified Party (including reasonable fees and expenses to the extent incurred in connection with investigating or preparing to defend such Proceeding in a manner not inconsistent with this Section) shall be paid to the Indemnified Party, as incurred, within ten Trading Days of written notice thereof to the Indemnifying Party (regardless of whether it is ultimately determined that an Indemnified Party is not entitled to indemnification hereunder; provided, that the Indemnifying Party may require such Indemnified Party to undertake to reimburse all such fees and expenses to the extent it is finally judicially determined that such Indemnified Party is not entitled to indemnification hereunder).

(d) Contribution.

(i) If a claim for indemnification under Section 5(a) or 5(b) is unavailable to an Indemnified Party (by reason of public policy or otherwise), then each Indemnifying Party, in lieu of indemnifying such Indemnified Party, shall contribute to the amount paid or payable by such Indemnified Party as a result of such Losses, in such proportion as is appropriate to reflect the relative fault of the Indemnifying Party and Indemnified Party in connection with the actions, statements or omissions that resulted in such Losses as well as any other relevant equitable considerations. The relative fault of such Indemnifying Party and Indemnified Party shall be determined by reference to, among other things, whether any action in question, including any untrue or alleged untrue statement of a material fact or omission or alleged omission of a material fact, has been taken or made by, or relates to information supplied by, such Indemnifying Party or Indemnified Party, and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such action, statement or omission. The amount paid or payable by a party as a result of any Losses shall be deemed to include, subject to the limitations set forth in Section 5(c), any reasonable attorneys’ or other reasonable fees or expenses incurred by such party in connection with any Proceeding to the extent such party would have been indemnified for such fees or expenses if the indemnification provided for in this Section was available to such party in accordance with its terms.

(ii) The parties hereto agree that it would not be just and equitable if contribution pursuant to this Section 5(d) were determined by pro rata allocation or by any other method of allocation that does not take into account the equitable considerations referred to in the immediately preceding paragraph. Notwithstanding the provisions of this Section 5(d), no Holder shall be required to contribute, in the aggregate, any amount in excess of the amount by which the proceeds actually received by such Holder from the sale of the Registrable Securities subject to the Proceeding exceeds the amount of any damages that such Holder has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of

such fraudulent misrepresentation.

(iii) The indemnity and contribution agreements contained in this Section are in addition to any liability that the Indemnifying Parties may have to the Indemnified Parties and are not in diminution or limitation of the indemnification provisions under the Purchase Agreement.

6. Miscellaneous.

(a) Remedies. In addition to being entitled to exercise all rights provided herein or granted by law, including recovery of damages, each Holder will be entitled to seek specific performance under this Agreement. The parties agree that monetary damages may not be adequate compensation for any loss incurred by reason of any breach of the Company’s obligations contained in this Agreement and the Company hereby agrees to waive and not to assert in any action for specific performance of any such obligation the defense that a remedy at law would be adequate.

(b) Reports. With a view to making available to the Holders the benefits of Rule 144 or any other similar rule or regulation of the Commission that may at any time permit the Holders to sell securities of the Company to the public without registration, the Company agrees, from the date hereof until the end of the Effectiveness Period, to (i) make and keep current public information available, as those terms are understood and defined in Rule 144; (ii) file with the Commission in a timely manner all reports and other documents required of the Company under the Securities Act and the Exchange Act; and (iii) furnish to each Holder so long as such Holder owns Registrable Securities, promptly upon request, (A) a written statement by the Company, if true, that it has complied with the reporting requirements of Rule 144, the Securities Act and the Exchange Act, (B) a copy of the most recent annual or quarterly report of the Company and such other reports and documents so filed by the Company, and (C) such other information as may be reasonably requested to permit the Holders to sell such securities pursuant to Rule 144 without registration.

(c) Subsequent Registration Rights. Until the Initial Registration Statement required hereunder is declared effective by the Commission, the Company shall not enter into any agreement granting any registration rights with respect to any of its securities to any Person without the written consent of the Purchaser, for so long as the Purchaser or its assignee holds any Registrable Securities.

(d) Discontinued Disposition. Each Holder agrees by its acquisition of such Registrable Securities that, upon receipt of a notice from the Company of the occurrence of any event of the kind described in clauses (ii) through (vi) of Section 3(c), such Holder will forthwith discontinue disposition of such Registrable Securities under the Registration Statement until such Holder’s receipt of the copies of the supplemented Prospectus and/or amended Registration Statement or until it is advised in writing by the Company that the use of the applicable Prospectus may be resumed, and, in either case, has received copies of any additional or supplemental filings that are incorporated or deemed to be incorporated by reference in such Prospectus or Registration Statement. The Company acknowledges that receipt of such notice

shall not require the Holders to cease to sell any Registrable Securities other than pursuant to the Registration Statement and that the Holders may at any time without restriction sell Registrable Securities other than pursuant to the Registration Statement.

(e) Furnishing of Information. Each Holder shall furnish in writing to the Company such information regarding itself, the Registrable Securities held by it and the intended method of disposition of the Registrable Securities held by it, as shall be reasonably requested by the Company to effect the registration of such Registrable Securities and shall execute such documents in connection with such registration as the Company may reasonably request.

(f) Piggy-Back Registrations. If at any time during the Effectiveness Period, except as contemplated by Section 2(b) or Section 2(d) hereof, there is not an effective Registration Statement covering all of the Registrable Securities and the Company shall determine to prepare and file with the Commission a registration statement relating to an offering for its own account or the account of others under the Securities Act of any of its equity securities, other than on Form S-4 or Form S-8 (each as promulgated under the Securities Act) or their then equivalents relating to equity securities to be issued solely in connection with any acquisition of any entity or business or equity securities issuable in connection with the stock option or other employee benefit plans, then the Company shall send to each Holder a written notice of such determination and, if within 15 days after the date of such notice, any such Holder shall so request in writing, the Company shall include in such registration statement all or any part of such Registrable Securities such Holder requests to be registered.

(g) Amendments and Waivers. No provision of this Agreement may be waived or amended except in a written instrument signed by the Company and the Purchaser, for so long as the Purchaser holds any Registrable Securities, and thereafter by the Holder(s) of a majority of the then outstanding Registrable Securities. Any amendment or waiver effected in accordance with this Section 6(g) shall be binding upon each Holder and the Company. No waiver of any default with respect to any provision, condition or requirement of this Agreement shall be deemed to be a continuing waiver in the future or a waiver of any subsequent default or a waiver of any other provision, condition or requirement hereof, nor shall any delay or omission of either party to exercise any right hereunder in any manner impair the exercise of any such right.

(h) Notices. Any and all notices or other communications or deliveries required or permitted to be provided hereunder shall be in writing and shall be deemed given and effective on the earliest of (a) the date of transmission, if such notice or communication is delivered via facsimile at the facsimile number set forth on the signature pages attached hereto prior to 5:30 p.m. (New York City time) on a Trading Day, (b) the next Trading Day after the date of transmission, if such notice or communication is delivered via facsimile at the facsimile number set forth on the signature pages attached hereto on a day that is not a Trading Day or later than 5:30 p.m. (New York City time) on any Trading Day, (c) the 2nd Trading Day following the date of mailing, if sent by U.S. nationally recognized overnight courier service, or (d) upon actual receipt by the party to whom such notice is required to be given. The addresses for such notices and communications shall be as follows:

If to the Company:

ImmunoGen, Inc.

830 Winter Street

Waltham, MA 02451

Fax: (781) 895-0613

Attn: General Counsel

with a copy (which shall not constitute notice) to:

Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
One Financial Center
Boston, MA 02111
Fax: (617) 542-2241

Attn: Jonathan L. Kravetz, Esq

If to the Purchaser:

Ziff Brothers Investments
350 Park Avenue
New York, NY 10022
Attention: General Counsel
Fax: (212) 292-6659

with a copy (which shall not constitute notice) to:

Fried, Frank, Harris, Shriver & Jacobson LLP
One New York Plaza
New York, New York 10004
Attention: Jonathan Adler, Esq.

Fax: (212) 859-4000

If to a Holder (other than the Purchaser), to such Holder’s address and facsimile number set forth in the agreement pursuant to which such Holder agrees to be bound by the provisions of this Agreement in accordance with Section 6(h);

or such other address as may be designated in writing hereafter, in the same manner, by such Person.

(i) Successors and Assigns. This Agreement shall inure to the benefit of and be binding upon the successors and permitted assigns of each of the parties and shall inure to the benefit of each Holder. The Company may not assign its obligations under this Agreement. Each Holder may assign any or all of its respective rights under this Agreement to any transferee of all or any portion of such Holder’s Registrable Securities provided the transferee or assignee agrees to be bound by all of the provisions contained herein. Each Holder shall, no later than 10 days following any transfer, give written notice (which shall include the identity of the transferee) to the Company of any transfer by it of Registrable Securities that remain Registrable Securities following such transfer.

(j) Execution and Counterparts. This Agreement may be executed in two or more counterparts, all of which when taken together shall be considered one and the same agreement and shall become effective when counterparts have been signed by each party and delivered to the other party, it being understood that both parties need not sign the same counterpart. In the event that any signature is delivered by facsimile transmission or by e-mail delivery of a “.pdf” format data file, such signature shall create a valid and binding obligation of the party executing (or on whose behalf such signature is executed) with the same force and effect as if such facsimile or “.pdf” signature page were an original thereof.

(k) Governing Law. All questions concerning the construction, validity, enforcement and interpretation of this Agreement shall be governed by and construed and enforced in accordance with the internal laws of the State of New York, without regard to the principles of conflicts of law thereof. Each party agrees that all legal proceedings concerning the interpretations, enforcement and defense of the transactions contemplated by this Agreement (whether brought against a party hereto or its respective affiliates, directors, officers, shareholders, employees or agents) shall be commenced exclusively in the state and federal courts sitting in the City of New York. Each party hereby irrevocably submits to the exclusive jurisdiction of the state and federal courts sitting in the City of New York, borough of Manhattan for the adjudication of any dispute hereunder or in connection herewith or with any transaction contemplated hereby or discussed herein, and hereby irrevocably waives, and agrees not to assert in any suit, action or proceeding, any claim that it is not personally subject to the jurisdiction of any such court, that such suit, action or proceeding is improper or is an inconvenient venue for such proceeding. Each party hereby irrevocably waives personal service of process and consents to process being served in any such suit, action or proceeding by mailing a copy thereof via registered or certified mail or overnight delivery (with evidence of delivery) to such party at the address in effect for notices to it under this Agreement and agrees that such service shall constitute good and sufficient service of process and notice thereof. Nothing contained herein shall be deemed to limit in any way any right to serve process in any other manner permitted by law. If either party shall commence an action or proceeding to enforce any provisions of this Agreement, then the prevailing party in such action or proceeding shall be reimbursed by the other party for its reasonable attorneys’ fees and other costs and expenses incurred with the investigation, preparation and prosecution of such action or proceeding.

(l) Cumulative Remedies. The remedies provided herein are cumulative and not exclusive of any remedies provided by law.

(m) Severability. If any term, provision, covenant or restriction of this Agreement is held by a court of competent jurisdiction to be invalid, illegal, void or unenforceable, the remainder of the terms, provisions, covenants and restrictions set forth herein shall remain in full force and effect and shall in no way be affected, impaired or invalidated, and the parties hereto shall use their commercially reasonable efforts to find and employ an alternative means to achieve the same or substantially the same result as that contemplated by such term, provision, covenant or restriction. It is hereby stipulated and declared to be the intention of the parties that they would have executed the remaining terms, provisions, covenants and restrictions without including any of such that may be hereafter declared invalid, illegal, void or unenforceable.

(n) Actions by Holders. All consents and other determinations required to be made by the Holders pursuant to this Agreement shall be made, unless otherwise specified in this Agreement, by the Purchaser, for so long as the Purchaser holds any Registrable Securities, and thereafter by the Holder(s) of a majority of the then outstanding Registrable Securities.

(o) Headings. The headings in this Agreement are for convenience of reference only and shall not limit or otherwise affect the meaning hereof.

[Signature Page Follows]

IN WITNESS WHEREOF, the parties have executed this Registration Rights Agreement as of the date first written above.

COMPANY:

	IMMUNOGEN, INC.


	By:	/s/ Mitchel Sayare
		Name: Mitchel Sayare, Ph.D.
		Title: President, Chief Executive Officer
		and Chairman of the Board


PURCHASER:

	ZIFF ASSET MANAGEMENT, L.P.

	By: PBK Holdings, Inc., its general partner


	By:	/s/ David Gray
		Name: David Gray
		Title: Vice President

[Signature Page to Registration Rights Agreement]

ANNEX A

PLAN OF DISTRIBUTION

We are registering the shares on behalf of the selling stockholder. The selling stockholder and any of its transferees who are affiliates, or any of its pledgees, assignees and successors-in-interest may, from time to time, sell any or all of its shares of Common Stock or any interests therein on any stock exchange, market or trading facility on which the shares are traded or in private transactions. These sales may be at market prices prevailing at the time of sale, at prices related to the prevailing market prices or at fixed or negotiated prices, or varying prices determined at the time of sale. The shares of Common Stock may be sold by the selling stockholder directly to one or more purchasers, through agents designated from time to time or to or through one or more underwriters or broker-dealers designated from time to time. In the event the shares of Common Stock are publicly offered through broker-dealers or agents, the selling stockholder may enter into agreements with respect thereto. The selling stockholder may also transfer, devise or gift these shares by other means not described in this prospectus. The selling stockholder may also use any one or more of the following methods when selling shares:

· on any national securities exchange or quotation service on which the securities may be listed or quoted at the time of sale or in private transactions;

· in the over-the-counter market;

· in transactions otherwise than on these exchanges or systems or in the over-the-counter market;

· ordinary brokerage transactions and transactions in which the broker-dealer solicits purchasers;

· block trades in which the broker-dealer will attempt to sell the shares as agent but may position and resell a portion of the block as principal to facilitate the transaction;

· purchases by a broker-dealer as principal and resale by the broker-dealer for its account;

· an exchange distribution in accordance with the rules of the applicable exchange;

· privately negotiated transactions;

· short sales;

· broker-dealers may agree with the selling stockholder to sell a specified number of such shares at a stipulated price per share;

· through the writing or settlement of options or other hedging transactions entered into after the effective date of the registration statement of which this prospectus is a part,

whether through an options exchange or otherwise;

· a combination of any such methods of sale; or

· any other method permitted pursuant to applicable law.

The selling stockholder may also sell shares under Rule 144 under the Securities Act, if available, rather than under this prospectus, provided it meets the criteria and conforms to the requirements of such Rule.

In connection with the sale of the Common Stock or interests therein, the selling stockholder may enter into hedging transactions with broker-dealers or other financial institutions, which may in turn engage in short sales of the Common Stock in the course of hedging the positions they assume. The selling stockholder may also enter into option or other transactions with broker-dealers or other financial institutions or the creation of one or more derivative securities which require the delivery to such broker-dealer or other financial institution of shares offered by this prospectus, which shares such broker-dealer or other financial institution may resell pursuant to this prospectus (as supplemented to reflect such transaction). The selling stockholder may also engage in short sales against the box, puts and calls, loans or pledges and other transactions in securities of the Company or derivatives of Company securities and may sell or deliver shares in connection with these trades.

Broker-dealers engaged by the selling stockholder may arrange for other broker-dealers to participate in sales. Broker-dealers may receive commissions or discounts from the selling stockholder (or, if any broker-dealer acts as agent for the purchaser of shares, from the purchaser) in amounts to be negotiated. The selling stockholder does not expect these commissions and discounts to exceed what is customary in the types of transactions involved. Discounts, commissions and similar selling expenses, if any, will be borne by the selling stockholder.

The selling stockholder may from time to time pledge or grant a security interest in some or all of the shares of Common Stock owned by it, and, if it defaults in the performance of its secured obligations, the pledgees or secured parties may offer and sell the shares of Common Stock from time to time under this prospectus, or under an amendment to this prospectus under Rule 424(b)(3) or other applicable provision of the Securities Act amending, if necessary, the list of selling stockholders to include the pledgee, transferee or other successors in interest as selling stockholders under this prospectus.

The selling stockholder may also transfer shares of Common Stock in other circumstances, in which case, upon notification of such transfer, we will file, to the extent required, a supplement to this prospectus disclosing all required information and the transferees, pledgees, assignees and successors-in-interest will be the selling beneficial owner for purposes of this prospectus and may sell the shares of Common Stock from time to time under this prospectus.

The selling stockholder and any broker-dealers or agents that are involved in selling the shares may be deemed to be “underwriters” within the meaning of Section 2(11) of the Securities

Act in connection with such sales. In such event, any commissions received by such broker-dealers or agents and any profit on the resale of the shares purchased by them may be deemed to be underwriting commissions or discounts under the Securities Act. The maximum commission or discount to be received by any member of the Financial Industry Regulatory Authority (FINRA) or independent broker-dealer will not be greater than 8% of the initial gross proceeds from the sale of any security being sold.

We have agreed to indemnify the selling stockholder against certain liabilities, including liabilities arising under the Securities Act. The selling stockholder may agree to indemnify any agent, dealer or broker-dealer that participates in transactions involving sales of the shares against certain liabilities, including liabilities arising under the Securities Act.

Because the selling stockholder may be deemed to be an “underwriter” within the meaning of Section 2(11) of the Securities Act, the selling stockholder will be subject to the prospectus delivery requirements of the Securities Act, which may include delivery through the facilities of Nasdaq. The Company has informed the selling stockholder that the anti-manipulative provisions of Regulation M promulgated under the Exchange Act may apply to its sales in the market.

The selling stockholder has advised us that, as of the date of this prospectus, it has not entered into any agreements, understandings or arrangements with any underwriters or broker-dealers regarding the sale of its securities, nor is there an underwriter or coordinating broker acting in connection with the proposed sale of shares by the selling stockholder. However, the selling stockholder may enter into agreements, understandings or arrangements with underwriters or broker-dealers regarding the sale of its securities and upon notification by the selling stockholder that any material arrangement has been entered into with a broker-dealer or underwriter for the sale of shares through a block trade, special offering, exchange distribution or secondary distribution or a purchase by a broker or dealer, we will file a supplement to this prospectus, if required, disclosing all required information. In addition, upon notification by the selling stockholder that a donee or pledgee intends to sell more than 500 shares, we will file a supplement to this prospectus.

The aggregate proceeds to the selling stockholder from the sale of the Common Stock offered by the selling stockholder will be the purchase price of the Common Stock less discounts or commissions, if any. The selling stockholder reserves the right to accept and, together with its agents from time to time, to reject, in whole or in part, any proposed purchase of Common Stock to be made directly or through agents. We will not receive any of the proceeds from this offering.

We will pay all fees and expenses incident to the registration of the shares, including up to $25,000 of the fees and disbursements of counsel to the selling stockholder.

We have agreed with the selling stockholder to keep the registration statement of which this prospectus constitutes a part effective until the earlier to occur of (x) such time as all of the shares covered by this prospectus have been disposed of pursuant to the registration statement or pursuant to Rule 144 under the Securities Act, or (y) following any sale (other than pursuant to the registration statement or Rule 144) of such shares by the selling stockholder to any person (other than an affiliate of the selling stockholder) who, after giving effect to such sale, owns less

iii

than 10% of our outstanding shares of Common Stock, at such time as such person is able to sell all of his or its shares pursuant to Rule 144(b)(1).

Immunogen (IMGN) 8-KEntry into a Material Definitive Agreement