IMMUNOGEN INC
S-3/A, EX-1.1, 2000-10-27
PHARMACEUTICAL PREPARATIONS
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                                                                     EXHIBIT 1.1


                                4,000,000 SHARES

                                 IMMUNOGEN, INC.

                           COMMON STOCK $.01 PAR VALUE

                             UNDERWRITING AGREEMENT
                             ----------------------

                                                              ____________, 2000

SG COWEN SECURITIES CORPORATION
ROBERTSON STEPHENS, INC.
ADAMS HARKNESS & HILL, INC.
  As Representatives of the several Underwriters
c/o SG Cowen Securities Corporation
Financial Square
New York, New York 10005

Dear Sirs:

1. INTRODUCTORY. ImmunoGen, Inc., a Massachusetts corporation (the "Company"),
proposes to sell, pursuant to the terms of this Agreement, to the several
underwriters named in Schedule A hereto (the "Underwriters," or, each, an
"Underwriter"), an aggregate of 4,000,000 shares of Common Stock, $.01 par value
(the "Common Stock") of the Company. The aggregate of 4,000,000 shares so
proposed to be sold is hereinafter referred to as the "Firm Stock". The Company
also proposes to sell to the Underwriters, upon the terms and conditions set
forth in Section 3 hereof, up to an additional 600,000 shares of Common Stock
(the "Optional Stock"). The Firm Stock and the Optional Stock are hereinafter
collectively referred to as the "Stock". SG Cowen Securities Corporation ("SG
Cowen") and Robertson Stephens, Inc. and Adams Harkness & Hill, Inc. are acting
as representatives of the several Underwriters and in such capacity are
hereinafter referred to as the "Representatives."

2. REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company represents and
warrants to, and agrees with, the several Underwriters that:

     (a) A registration statement on Form S-3 (File No. 333-48042) (the "Initial
     Registration Statement") in respect of the Stock has been filed with the
     Securities and Exchange Commission (the "Commission"); the Initial
     Registration Statement and any post-effective amendment thereto, each in
     the form heretofore delivered to you, and, excluding exhibits thereto but
     including all documents incorporated by reference in the prospectus
     contained therein, to you for each of the other Underwriters, have been
     declared effective by the Commission in such form; other than a
     registration statement, if any, increasing the size of the offering (a
     "Rule 462(b) Registration Statement"), filed pursuant to Rule 462(b) under
     the Securities Act of 1933, as amended (the "Securities Act") and the rules
     and regulations (the "Rules and Regulations") of the Commission thereunder,
     which became effective upon filing, no other document with respect to the
     Initial Registration Statement or document incorporated by reference
     therein has heretofore been filed with the Commission; and no stop order
     suspending the effectiveness of the Initial Registration Statement, any
     post-effective amendment thereto or the Rule 462(b) Registration Statement,
     if any, has been issued and no proceeding for that purpose has been
     initiated or threatened by the Commission (any preliminary prospectus
     included in the Initial Registration Statement or filed with the Commission
     pursuant to Rule 424(a) of the Rules and Regulations, is hereinafter called
     a "Preliminary Prospectus"); the various parts of the Initial Registration


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     Statement and the Rule 462(b) Registration Statement, if any, including all
     exhibits thereto and including (i) the information contained in the form of
     final prospectus filed with the Commission pursuant to Rule 424(b) under
     the Securities Act and deemed by virtue of Rule 430A under the Securities
     Act to be part of the Initial Registration Statement at the time it was
     declared effective and (ii) the documents incorporated by reference in the
     prospectus contained in the Initial Registration Statement at the time such
     part of the Initial Registration Statement became effective, each as
     amended at the time such part of the Initial Registration Statement became
     effective or such part of the Rule 462(b) Registration Statement, if any,
     became or hereafter becomes effective, are hereinafter collectively called
     the "Registration Statements"; such final prospectus, in the form first
     filed pursuant to Rule 424(b) under the Securities Act, is hereinafter
     called the "Prospectus"; and any reference herein to any Preliminary
     Prospectus or the Prospectus shall be deemed to refer to and include the
     documents incorporated by reference therein pursuant to Item 12 of Form S-3
     under the Securities Act, as of the date of such Preliminary Prospectus or
     Prospectus, as the case may be; any reference to any amendment or
     supplement to any Preliminary Prospectus or the Prospectus shall be deemed
     to refer to and include any documents filed after the date of such
     Preliminary Prospectus or Prospectus, as the case may be, under the
     Securities Exchange Act of 1934, as amended (the "Exchange Act"), and
     incorporated by reference in such Preliminary Prospectus or Prospectus, as
     the case may be; and any reference to any amendment to the Registration
     Statements shall be deemed to refer to and include any annual report of the
     Company filed pursuant to Section 13(a) or 15(d) of the Exchange Act after
     the effective date of the Initial Registration Statement that is
     incorporated by reference in the Registration Statements. No document has
     been or will be prepared or distributed in reliance on Rule 434 under the
     Securities Act. No order preventing or suspending the use of any
     Preliminary Prospectus has been issued by the Commission.

     (b) The Initial Registration Statement complies (and the Rule 462(b)
     Registration Statement, if any, the Prospectus and any amendments or
     supplements to either of the Registration Statements or the Prospectus,
     when they become effective or are filed with the Commission, as the case
     may be, will comply) as to form in all material respects to the
     requirements of the Securities Act and the Rules and Regulations and do not
     and will not, as of the applicable effective date (as to the Registration
     Statements and any amendment thereto) and as of the applicable filing date
     (as to the Prospectus and any amendment or supplement thereto) 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 in which they were made, not misleading;
     PROVIDED, however, that the foregoing representations and warranties shall
     not apply to information contained in or omitted from the Registration
     Statements or the Prospectus or any such amendment or supplement thereto in
     reliance upon, and in conformity with, written information furnished to the
     Company through the Representatives by or on behalf of any Underwriter
     specifically for inclusion therein.

     (c) The documents incorporated by reference in the Prospectus, when they
     became effective or were filed with the Commission, as the case may be,
     complied as to form in all material respects to the requirements of the
     Securities Act or the Exchange Act, as applicable, and the rules and
     regulations of the Commission thereunder, and none of such documents
     contained any untrue statement of a material fact or omitted to state any
     material fact required to be stated therein or necessary to make the
     statements therein, in light of the circumstances in which they were made,
     not misleading; and any further documents so filed and incorporated by
     reference in the Prospectus, when such documents become effective or are
     filed with Commission, as the case may be, will comply as to form in all
     material respects to the requirements of the Securities Act or the Exchange
     Act, as applicable, and the rules and regulations of the Commission
     thereunder and will not contain any untrue statement of a material fact or
     omit to state any material fact required to be stated therein, in light of
     the circumstances in which they were made, or necessary to make the
     statements therein not misleading.


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     (d) The Company and each of its subsidiaries (as defined in Section 14)
     have been duly incorporated and are validly existing as corporations in
     good standing under the laws of their respective jurisdictions of
     incorporation, are duly qualified to do business and are in good standing
     as foreign corporations in each jurisdiction in which their respective
     ownership or lease of property or the conduct of their respective
     businesses requires such qualification, and have all power and authority
     necessary to own or hold their respective properties and to conduct the
     businesses in which they are engaged, except where the failure to so
     qualify or have such power or authority would not have, singularly or in
     the aggregate, a material adverse effect on the condition (financial or
     otherwise), results of operations, business or prospects of the Company and
     its subsidiaries taken as a whole (a "Material Adverse Effect"). The
     Company owns or controls, directly or indirectly, only the following
     corporations, associations or other entities: (i) Immunogen Securities
     Corp., a wholly-owned subsidiary, and (ii) Apoptosis Technology, Inc., of
     which the Company owns _____% of the outstanding common stock and _____% of
     the outstanding preferred stock of Apoptosis Technology, Inc.

     (e) This Agreement has been duly authorized executed and delivered by the
     Company.

     (f) The Stock to be issued and sold by the Company to the Underwriters
     hereunder has been duly and validly authorized and, when issued and
     delivered against payment therefor as provided herein, will be duly and
     validly issued, fully paid and nonassessable and free of any preemptive or
     similar rights and will conform to the description thereof contained in the
     Prospectus.

     (g) The Company has an authorized capitalization as set forth in the
     Prospectus, and all of the issued shares of capital stock of the Company
     have been duly and validly authorized and issued, are fully paid and
     non-assessable and conform to the description thereof contained in the
     Prospectus.

     (h) All the outstanding shares of capital stock of each subsidiary of the
     Company have been duly authorized and validly issued, are fully paid and
     nonassessable and, except to the extent set forth in the Prospectus, are
     owned by the Company directly or indirectly through one or more
     wholly-owned subsidiaries, free and clear of any claim, lien, encumbrance,
     security interest, restriction upon voting or transfer or any other claim
     of any third party.

     (i) The execution, delivery and performance of this Agreement by the
     Company and the consummation of the transactions contemplated hereby will
     not conflict with or result in a breach or violation of any of the terms or
     provisions of, or constitute a default under, any indenture, mortgage, deed
     of trust, loan agreement or other agreement or instrument to which the
     Company or any of its subsidiaries is a party or by which the Company or
     any of its subsidiaries is bound or to which any of the property or assets
     of the Company or any of its subsidiaries is subject, except for such
     conflicts, breaches, violations or defaults which would not, singularly or
     in the aggregate, have a Materially Adverse Effect, nor will such actions
     result in any violation of (x) the provisions of the charter or by-laws of
     the Company or any of its subsidiaries or (y) any statute or any order,
     rule or regulation of any court or governmental agency or body having
     jurisdiction over the Company or any of its subsidiaries or any of their
     properties or assets, except with respect to clause (y) for such violations
     which would not, singularly or in the aggregate, have a Material Adverse
     Effect.

     (j) Except for the registration of the Stock under the Securities Act and
     such consents, approvals, authorizations, registrations or qualifications
     as may be required under the Exchange Act and applicable state securities
     laws or the securities laws of any foreign


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     jurisdiction in connection with the purchase and distribution of the Stock
     by the Underwriters, no consent, approval, authorization or order of, or
     filing or registration with, any such court or governmental agency or body
     is required for the execution, delivery and performance of this Agreement
     by the Company and the consummation of the transactions contemplated
     hereby.

     (k) PricewaterhouseCoopers LLP, who have expressed their opinions on the
     audited financial statements included or incorporated by reference in the
     Registration Statements and the Prospectus are independent public
     accountants as required by the Securities Act and the Rules and
     Regulations.

     (l) The consolidated financial statements, together with the related notes
     included or incorporated by reference in the Prospectus and in each
     Registration Statement fairly present, in all material respects, the
     financial position and the results of operations and changes in financial
     position of the Company and its consolidated subsidiaries at the respective
     dates or for the respective periods therein specified. Such statements and
     related notes have been prepared in accordance with generally accepted
     accounting principles applied on a consistent basis except as may be set
     forth in the Prospectus.

     (m) Neither the Company nor any of its subsidiaries has sustained, since
     the date of the latest audited financial statements included or
     incorporated by reference in the Prospectus, any loss or interference with
     its business from fire, explosion, flood or other calamity, whether or not
     covered by insurance, or from any labor dispute or court or governmental
     action, order or decree, which, singularly or in the aggregate, would
     reasonably be expected to have a Material Adverse Effect otherwise than as
     set forth or contemplated in the Prospectus; and, since such date, there
     has not been any change in the capital stock or long-term debt of the
     Company or any of its subsidiaries or any adverse change, or any
     development involving a prospective adverse change, in or affecting the
     business, general affairs, management, financial position, stockholders'
     equity or results of operations of the Company and its subsidiaries taken
     as a whole, which, singularly or in the aggregate, would reasonably be
     expected to have a Material Adverse Effect otherwise than as set forth or
     contemplated in the Prospectus.

     (n) Except as set forth in the Prospectus, there is no legal or
     governmental proceeding pending to which the Company or any of its
     subsidiaries is a party or of which any property or assets of the Company
     or any of its subsidiaries is the subject which, singularly or in the
     aggregate, if determined adversely to the Company or any of its
     subsidiaries, might have a Material Adverse Effect or would prevent or
     adversely affect the ability of the Company to perform its obligations
     under this Agreement; and to the best of the Company's knowledge, no such
     proceedings are threatened or contemplated by governmental authorities or
     threatened by others.

     (o) Neither the Company nor any of its subsidiaries (i) is in violation of
     its charter or by-laws, (ii) is in default in any respect, and no event has
     occurred which, with notice or lapse of time or both, would constitute such
     a default, in the due performance or observance of any term, covenant or
     condition contained in any indenture, mortgage, deed of trust, loan
     agreement or other agreement or instrument to which it is a party or by
     which it is bound or to which any of its property or assets is subject or
     (iii) is in violation in any respect of any law, ordinance, governmental
     rule, regulation or court decree to which it or its property or assets may
     be subject except with respect to clauses (ii) and (iii) for any violations
     or defaults which, singularly or in the aggregate, would not have a
     Material Adverse Effect.

     (p) The Company and each of its subsidiaries possess all licenses,
     certificates, authorizations and permits issued by, and have made all
     declarations and filings with, the appropriate state, federal or foreign
     regulatory agencies or bodies which are necessary or


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     desirable for the ownership of their respective properties or the conduct
     of their respective businesses as described in the Prospectus except where
     any failures to possess or make the same, singularly or in the aggregate,
     would not have a Material Adverse Effect, and the Company has not received
     notification of any revocation or modification of any such license,
     authorization or permit and has no reason to believe that any such license,
     certificate, authorization or permit will not be renewed.

     (q) Neither the Company nor any of its subsidiaries is or, after giving
     effect to the offering of the Stock and the application of the proceeds
     thereof as described in the Prospectus will become an "investment company"
     within the meaning of the Investment Company Act of 1940, as amended and
     the rules and regulations of the Commission thereunder.

     (r) Neither the Company nor any of its officers, directors or affiliates
     has taken or will take, directly or indirectly, any action designed or
     intended to stabilize or manipulate the price of any security of the
     Company, or which caused or resulted in, or which might in the future
     reasonably be expected to cause or result in, stabilization or manipulation
     of the price of any security of the Company.

     (s) The Company and its subsidiaries own or possess the right to use all
     patents, trademarks, trademark registrations, service marks, service mark
     registrations, trade names, copyrights, licenses, inventions, trade secrets
     and rights described in the Prospectus as being owned by them for the
     conduct of their respective businesses, and the Company is not aware of any
     claim to the contrary or any challenge by any other person to the rights of
     the Company and its subsidiaries with respect to the foregoing. The
     Company's business as now conducted and as proposed to be conducted does
     not and will not infringe or conflict with any patents, trademarks, service
     marks, trade names, copyrights, trade secrets, licenses or other
     intellectual property or franchise right of any person, in a manner which,
     singularly or in the aggregate, would reasonably be expected to have a
     Material Adverse Effect. Except as described in the Prospectus, the Company
     has not received notice of any claim alleging the infringement by the
     Company of any patent, trademark, service mark, trade name, copyright,
     trade secret, license in or other intellectual property right or franchise
     right of any person.

     (t) The Company and each of its subsidiaries have good and marketable title
     in fee simple to, or have valid rights to lease or otherwise use, all items
     of real or personal property which are material to the business of the
     Company and its subsidiaries taken as a whole, in each case free and clear
     of all liens, encumbrances, claims and defects that may result in a
     Material Adverse Effect.

     (u) No labor disturbance by the employees of the Company or any of its
     subsidiaries exists or, to the best of the Company's knowledge, is imminent
     which might be expected to have a Material Adverse Effect. The Company is
     not aware that any key employee or significant group of employees of the
     Company or any subsidiary plans to terminate employment with the Company or
     any such subsidiary.

     (v) No "prohibited transaction" (as defined in Section 406 of the Employee
     Retirement Income Security Act of 1974, as amended, including the
     regulations and published interpretations thereunder ("ERISA"), or Section
     4975 of the Internal Revenue Code of 1986, as amended from time to time
     (the "Code")) or "accumulated funding deficiency" (as defined in Section
     302 of ERISA) or any of the events set forth in Section 4043(b) of ERISA
     (other than events with respect to which the 30-day notice requirement
     under Section 4043 of ERISA has been waived) has occurred with respect to
     any employee benefit plan maintained by the Company which could have a
     Material Adverse Effect; each employee benefit plan maintained by the
     Company is in compliance in all material respects with applicable law,
     including ERISA and the


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     Code; the Company has not incurred and does not expect to incur liability
     under Title IV of ERISA with respect to the termination of, or withdrawal
     from, any "pension plan"; and each "pension plan" (as defined in Section
     3(2) of ERISA) maintained by the Company would have any liability that is
     intended to be qualified under Section 401(a) of the Code is so qualified
     in all material respects and nothing has occurred, whether by action or by
     failure to act, which could cause the loss of such qualification (other
     than changes in the Code requirements applicable to qualified plans which
     may require amendments to any pension plan(s) maintained by the Company and
     for which the applicable remedial amendment period has not expired).

     (w) There has been no storage, generation, transportation, handling,
     treatment, disposal, discharge, emission, or other release of any kind of
     toxic or other wastes or other hazardous substances by, due to, or caused
     by the Company or any of its subsidiaries (or, to the best of the Company's
     knowledge, any other entity for whose acts or omissions the Company or any
     of its subsidiaries is or may be liable) upon any of the property now or
     previously owned or leased by the Company or any of its subsidiaries, or
     upon any other property, in violation of any statute or any ordinance,
     rule, regulation, order, judgment, decree or permit or which would, under
     any statute or any ordinance, rule (including rule of common law),
     regulation, order, judgment, decree or permit, give rise to any liability,
     except for any violation or liability which would not have, singularly or
     in the aggregate with all such violations and liabilities, a Material
     Adverse Effect; there has been no disposal, discharge, emission or other
     release of any kind onto such property or into the environment surrounding
     such property of any toxic or other wastes or other hazardous substances
     with respect to which the Company or any of its subsidiaries have
     knowledge, except for any such disposal, discharge, emission, or other
     release of any kind which would not have, singularly or in the aggregate
     with all such discharges and other releases, a Material Adverse Effect.

     (x) The Company and its subsidiaries each (i) have filed with all necessary
     federal, state and foreign income and franchise tax returns, (ii) have paid
     all federal state, local and foreign taxes due and payable for which it is
     liable, and (iii) do not have any tax deficiency or claims outstanding or
     assessed or, to the best of the Company's knowledge, proposed against it
     which could reasonably be expected to have a Material Adverse Effect.

     (y) The Company and each of its subsidiaries carry, or are covered by,
     insurance in such amounts and covering such risks as the Company believes
     to be customary for companies engaged in similar businesses in similar
     industries.

     (z) The Company and each of its subsidiaries maintains a system of internal
     accounting controls sufficient to provide reasonable assurances that (i)
     transactions are executed in accordance with management's general or
     specific authorization; (ii) transactions are recorded as necessary to
     permit preparation of financial statements in conformity with generally
     accepted accounting principles and to maintain accountability for assets;
     (iii) access to assets is permitted only in accordance with management's
     general or specific authorization; and (iv) the recorded accountability for
     assets is compared with existing assets at reasonable intervals and
     appropriate action is taken with respect to any differences.

     (aa) The minute books of the Company and each of its subsidiaries have been
     made available to the Underwriters and counsel for the Underwriters, and
     such books (i) contain a complete summary of all meetings and actions of
     the directors and shareholders of the Company and each of its subsidiaries
     since the time of its respective incorporation through the date of the
     latest meeting and action, and (ii) accurately in all material respects
     reflect all transactions referred to in such minutes.


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     (bb) There is no franchise, lease, contract, agreement or document required
     by the Securities Act or by the Rules and Regulations to be described in
     the Prospectus or to be filed as an exhibit to the Registration Statements
     which is not described or filed therein as required.

     (cc) To the best of the Company's knowledge, no relationship, direct or
     indirect, exists between or among the Company on the one hand, and the
     directors, officers, stockholders, customers or suppliers of the Company on
     the other hand, which is required to be described in the Prospectus and
     which is not so described.

     (dd) No person or entity has the right to require registration of shares of
     Common Stock or other securities of the Company because of the filing or
     effectiveness of the Registration Statements or otherwise, except for
     persons and entities who have expressly waived such right or who have been
     given proper notice and have failed to exercise such right within the time
     or times required under the terms and conditions of such right or who have
     been given an "underwriter cut-back" notice in accordance with the terms of
     the applicable agreement under which such registration rights arise.

     (ee) Neither the Company nor any of its subsidiaries own any "margin
     securities" as that term is defined in Regulation U of the Board of
     Governors of the Federal Reserve System (the "Federal Reserve Board"), and
     none of the proceeds of the sale of the Stock will be used, directly or
     indirectly, for the purpose of purchasing or carrying any margin security,
     for the purpose of reducing or retiring any indebtedness which was
     originally incurred to purchase or carry any margin security or for any
     other purpose which might cause any of the Securities to be considered a
     "purpose credit" within the meanings of Regulation T, U or X of the Federal
     Reserve Board.

     (ff) Neither the Company nor any of its subsidiaries is a party to any
     contract, agreement or understanding with any person that would give rise
     to a valid claim against the Company or the Underwriters for a brokerage
     commission, finder's fee or like payment in connection with the offering
     and sale of the Stock.

     (gg) No forward-looking statement (within the meaning of Section 27A of the
     Securities Act and Section 21E of the Exchange Act) contained in the
     Prospectus has been made or reaffirmed without a reasonable basis or has
     been disclosed other than in good faith.

     (hh) The Stock is listed on the NASDAQ Stock Market's National Market.

3. PURCHASE SALE AND DELIVERY OF OFFERED SECURITIES. On the basis of the
representations, warranties and agreements herein contained, but subject to the
terms and conditions herein set forth, the Company agrees to sell to each
Underwriter, and each Underwriter agrees, severally and not jointly, to purchase
from the Company that number of shares of Firm Stock set forth opposite the name
of such Underwriter in Schedule A hereto.

The purchase price per share to be paid by the Underwriters to the Company for
the Stock will be $_____ per share (the "Purchase Price").

The Company will deliver the Firm Stock to the Representatives for the
respective accounts of the several Underwriters (in the form of definitive
certificates, issued in such names and in such denominations as the
Representatives may direct by notice in writing to the Company given at or prior
to 12:00 Noon, New York time, on the second full business day preceding the
First Closing Date (as defined below) against payment of the aggregate Purchase
Price therefor by wire transfer to an account at a bank acceptable to SG Cowen,
payable to the order of the Company, all at the offices of Shearman & Sterling,
located at 599 Lexington Avenue, New York, New York. Time shall be of the
essence, and delivery at the time and


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place specified pursuant to this Agreement is a further condition of the
obligations of each Underwriter hereunder. The time and date of the delivery and
closing shall be at 10:00 A.M., New York time, on __________, 2000, in
accordance with Rule 15c6-1 of the Exchange Act. The time and date of such
payment and delivery are herein referred to as the "First Closing Date". The
First Closing Date and the location of delivery of, and the form of payment for,
the Firm Stock may be varied by agreement between the Company and SG Cowen.

The Company shall make the certificates for the Stock available to the
Representatives for examination on behalf of the Underwriters in New York, New
York at least twenty-four hours prior to the First Closing Date.

For the purpose of covering any over-allotments in connection with the
distribution and sale of the Firm Stock as contemplated by the Prospectus, the
Underwriters may purchase all or less than all of the Optional Stock. The price
per share to be paid for the Optional Stock shall be the Purchase Price. The
Company agrees to sell to the Underwriters the number of shares of Optional
Stock specified in the written notice by SG Cowen described below and the
Underwriters agree, severally and not jointly, to purchase such shares of
Optional Stock. Such shares of Optional Stock shall be purchased from the
Company for the account of each Underwriter in the same proportion as the number
of shares of Firm Stock set forth opposite such Underwriter's name bears to the
total number of shares of Firm Stock (subject to adjustment by SG Cowen to
eliminate fractions). The option granted hereby may be exercised as to all or
any part of the Optional Stock at any time, and from time to time, not more than
thirty (30) days subsequent to the date of this Agreement. No Optional Stock
shall be sold and delivered unless the Firm Stock previously has been, or
simultaneously is, sold and delivered. The right to purchase the Optional Stock
or any portion thereof may be surrendered and terminated at any time upon notice
by SG Cowen to the Company.

The option granted hereby may be exercised by written notice being given to the
Company by SG Cowen setting forth the number of shares of the Optional Stock to
be purchased by the Underwriters and the date and time for delivery of and
payment for the Optional Stock. Each date and time for delivery of and payment
for the Optional Stock (which may be the First Closing Date, but not earlier) is
herein called the "Option Closing Date" and shall in no event be earlier than
two (2) business days nor later than five (5) business days after written notice
is given. (The Option Closing Date and the First Closing Date are herein called
the "Closing Dates".)

The Company will deliver the Optional Stock to the Underwriters (in the form of
definitive certificates, issued in such names and in such denominations as the
Representatives may direct by notice in writing to the Company given at or prior
to 12:00 Noon, New York time, on the second full business day preceding the
Option Closing Date against payment of the aggregate Purchase Price therefor in
federal (same day) funds by certified or official bank check or checks or wire
transfer to an account at a bank acceptable to SG Cowen payable to the order of
the Company all at the offices of Shearman & Sterling, located at 599 Lexington
Avenue, New York, New York. Time shall be of the essence, and delivery at the
time and place specified pursuant to this Agreement is a further condition of
the obligations of each Underwriter hereunder. The Company shall make the
certificates for the Optional Stock available to the Representatives for
examination on behalf of the Underwriters in New York, New York not later than
10:00 A.M., New York Time, on the business day preceding the Option Closing
Date. The Option Closing Date and the location of delivery of, and the form of
payment for, the Optional Stock may be varied by agreement between the Company
and SG Cowen.

The several Underwriters propose to offer the Stock for sale upon the terms and
conditions set forth in the Prospectus.

4. FURTHER AGREEMENTS OF THE COMPANY. The Company agrees with the several
Underwriters that:


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     (a) The Company will prepare the Rule 462(b) Registration Statement, if
     necessary, in a form approved by the Representatives and file such Rule
     462(b) Registration Statement with the Commission on the date hereof;
     prepare the Prospectus in a form approved by the Representatives and file
     such Prospectus pursuant to Rule 424(b) under the Securities Act not later
     than the second business day following the execution and delivery of this
     Agreement; make no further amendment or any supplement to the Registration
     Statements or to the Prospectus prior to the Option Closing Date to which
     the Representatives shall reasonably object by notice to the Company after
     a reasonable period to review; advise the Representatives, promptly after
     it receives notice thereof, of the time when any amendment to either
     Registration Statement has been filed or becomes effective or any
     supplement to the Prospectus or any amended Prospectus has been filed and
     to furnish the Representatives with copies thereof; file promptly all
     reports and any definitive proxy or information statements required to be
     filed by the Company with the Commission pursuant to Section 13(a), 13(c),
     14 or 15(d) of the Exchange Act subsequent to the date of the Prospectus
     and for so long as the delivery of a prospectus is required in connection
     with the offering or sale of the Stock; advise the Representatives,
     promptly after it receives notice thereof, of the issuance by the
     Commission of any stop order or of any order preventing or suspending the
     use of any Preliminary Prospectus or the Prospectus, of the suspension of
     the qualification of the Stock for offering or sale in any jurisdiction, of
     the initiation or threatening of any proceeding for any such purpose, or of
     any request by the Commission for the amending or supplementing of the
     Registration Statements or the Prospectus or for additional information;
     and, in the event of the issuance of any stop order or of any order
     preventing or suspending the use of any Preliminary Prospectus or the
     Prospectus or suspending any such qualification, use promptly its best
     efforts to obtain its withdrawal.

     (b) If at any time prior to the expiration of nine months after the
     effective date of the Initial Registration Statement when a prospectus
     relating to the Stock is required to be delivered any event occurs as a
     result of which the Prospectus as then amended or supplemented would
     include any untrue statement of a material fact, or omit to state any
     material fact necessary to make the statements therein, in light of the
     circumstances under which they were made, not misleading, or if it is
     necessary at any time to amend the Prospectus or to file under the Exchange
     Act any document incorporated by reference in the Prospectus to comply with
     the Securities Act or the Exchange Act, the Company will promptly notify
     the Representatives thereof and upon their request will prepare an amended
     or supplemented Prospectus or make an appropriate filing pursuant to
     Section 13 or 14 of the Exchange Act which will correct such statement or
     omission or effect such compliance. The Company will furnish without charge
     to each Underwriter and to any dealer in securities as many copies as the
     Representatives may from time to time reasonably request of such amended or
     supplemented Prospectus; and in case any Underwriter is required to deliver
     a prospectus relating to the Stock nine months or more after the effective
     date of the Initial Registration Statement, the Company upon the request of
     the Representatives and at the expense of such Underwriter will prepare
     promptly an amended or supplemented Prospectus as may be necessary to
     permit compliance with the requirements of Section 10(a)(3) of the
     Securities Act.

     (c) To furnish promptly to each of the Representatives and to counsel for
     the Underwriters a signed copy of each of the Registration Statements as
     originally filed with the Commission, and each amendment thereto filed with
     the Commission, including all consents and exhibits filed therewith.

     (d) To deliver promptly to the Representatives in New York City such number
     of the following documents as the Representatives shall reasonably request:
     (i) conformed copies of the Registration Statements as originally filed
     with the Commission and each amendment thereto (in each case excluding
     exhibits), (ii) each Preliminary Prospectus, (iii) the Prospectus (not
     later than


                                       9


<PAGE>


     10:00 A.M., New York time, of the business day following the execution and
     delivery of this Agreement) and any amended or supplemented Prospectus (not
     later than 10:00 A.M., New York City time, on the business day following
     the date of such amendment or supplement) (but if any Underwriter is
     required to deliver a prospectus relating to the Stock nine months or more
     after the effective date of the Initial Registration Statement, the Company
     shall provide such number of the Prospectus and any amendment or supplement
     Prospectus as the Representatives may request at their expense) and (iv)
     any document incorporated by reference in the Prospectus (excluding
     exhibits thereto).

     (e) To make generally available to its shareholders as soon as practicable,
     but in any event not later than eighteen months after the effective date of
     the Initial Registration Statement (as defined in Rule 158(c) under the
     Securities Act), an earnings statement of the Company and its subsidiaries
     (which need not be audited) complying with Section 11(a) of the Securities
     Act and the Rules and Regulations (including, at the option of the Company,
     Rule 158).

     (f) The Company will promptly take from time to time such actions as the
     Representatives may reasonably request to qualify the Stock for offering
     and sale under the securities or Blue Sky laws of such jurisdictions as the
     Representatives may designate and to continue such qualifications in effect
     for so long as required for the distribution of the Stock; PROVIDED that
     the Company and its subsidiaries shall not be obligated to qualify as
     foreign corporations in any jurisdiction in which they are not so qualified
     or to file a general consent to service of process in any jurisdiction;

     (g) During the period of five years from the date hereof, the Company will
     deliver to the Representatives and, upon request, to each of the other
     Underwriters, (i) as soon as they are available, copies of all reports or
     other communications furnished to shareholders and (ii) as soon as they are
     available, copies of any reports and financial statements furnished or
     filed with the Commission pursuant to the Exchange Act or any national
     securities exchange or automatic quotation system on which the Stock is
     listed or quoted.

     (h) The Company will not directly or indirectly offer, sell, assign,
     transfer, pledge, contract to sell, or otherwise dispose of any shares of
     Common Stock or securities convertible into or exercisable or exchangeable
     for Common Stock for a period of 90 days from the date of the Prospectus
     without the prior written consent of SG Cowen other than the Company's sale
     of the Stock hereunder, the issuance of shares pursuant to employee benefit
     plans, qualified stock option plans or other employee compensation plans
     existing on the date hereof or pursuant to currently outstanding options,
     warrants or rights and the issuance of shares in respect of the acquisition
     by the Company of the assets, capital stock or business of another person
     or entity whether by merger, exchange of stock or otherwise, or in
     connection with the entering into of a collaboration agreement with an
     unaffiliated third party; provided, however, that in any such case, it
     shall be a condition to such issuance of Common Stock to a third party that
     the third party execute an agreement stating that such third party is
     receiving and holding the Common Stock subject to the provisions of a
     Lockup Letter, as defined below, and there shall be no further transfer of
     such Common Stock except in accordance with the Lockup Letter. The Company
     will cause each officer, director and shareholder listed in Schedule B to
     furnish to the Representatives, prior to the First Closing Date, a letter,
     substantially in the form of Exhibit I hereto (the "Lockup Letter"),
     pursuant to which each such person shall agree not to directly or
     indirectly offer, sell, assign, transfer, pledge, contract to sell, or
     otherwise dispose of any shares of Common Stock or securities convertible
     into or exercisable or exchangeable for Common Stock for a period of 90
     days from the date of the Prospectus, without the prior written consent of
     SG Cowen, except that such persons may transfer any or all of their Common
     Stock subject to the Lockup Letter (a) by gift, will or intestacy, (b) to
     such person's partners, shareholders, members or affiliates, and (c) if
     such person is an individual, to such person's immediate family or to a
     trust, the beneficiaries of


                                       10


<PAGE>


     which are exclusively such person and/or members of such person's immediate
     family; provided, however, that in any such case it shall be a condition to
     the transfer that the transferee execute an agreement stating that the
     transferee is receiving and holding the Common Stock subject to the
     provisions of the Lockup Letter, and there shall be no further transfer of
     such Common Stock except in accordance with the Lockup Letter.

     (i) The Company will supply the Representatives with copies of all
     correspondence to and from, and all documents issued to and by, the
     Commission in connection with the registration of the Stock under the
     Securities Act.

     (j) Prior to each of the Closing Dates the Company will furnish to the
     Representatives, as soon as they have been prepared, copies of any
     unaudited interim consolidated financial statements of the Company for any
     periods subsequent to the periods covered by the financial statements
     appearing in the Registration Statements and the Prospectus.

     (k) Prior to each of the Closing Dates, the Company will not issue any
     press release or other communication directly or indirectly or hold any
     press conference with respect to the Company, its condition, financial or
     otherwise, or earnings, business affairs or business prospects (except for
     routine press releases or other communications in the ordinary course of
     business and consistent with the past practices of the Company and of which
     the Representatives are notified), without the prior written consent of the
     Representatives, unless in the judgment of the Company and its counsel, and
     after notification to the Representatives, such press release or
     communication is required by law.

     (l) In connection with the offering of the Stock, until SG Cowen shall have
     notified the Company of the completion of the resale of the Stock, the
     Company will not, and will cause its affiliated purchasers (as defined in
     Regulation M under the Exchange Act) not to, either alone or with one or
     more other persons, bid for or purchase, for any account in which it or any
     of its affiliated purchasers has a beneficial interest, any Stock, or
     attempt to induce any person to purchase any Stock; and not to, and to
     cause its affiliated purchasers not to, make bids or purchase for the
     purpose of creating actual, or apparent, active trading in or of raising
     the price of the Stock.

     (m) The Company will not take any action prior to the Option Closing Date
     which would require the Prospectus to be amended or supplemented pursuant
     to Section 4(b);

     (n) The Company will apply the net proceeds from the sale of the Stock as
     set forth in the Prospectus under the heading "Use of Proceeds".

5. PAYMENT OF EXPENSES. The Company agrees with the Underwriter to pay (a) the
costs incident to the authorization, issuance, sale, preparation and delivery of
the Stock and any taxes payable in that connection; (b) the costs incident to
the Registration of the Stock under the Securities Act; (c) subject to the
provisions of Section 4(b) and (d), the costs incident to the preparation,
printing and distribution of the Registration Statements, Preliminary
Prospectus, Prospectus any amendments and exhibits thereto or any document
incorporated by reference therein, the costs of printing, reproducing and
distributing the "Agreement Among Underwriters" between the Representatives and
the Underwriters, the Master Selected Dealers' Agreement, the Underwriters'
Questionnaire and this Agreement by mail, telex or other means of
communications; (d) the fees and expenses incurred in connection with filings
made with the National Association of Securities Dealers; (e) any applicable
listing or other fees; (f) the fees and expenses of qualifying the Stock under
the securities laws of the several jurisdictions as provided in Section 4(f) and
of preparing, printing and distributing Blue Sky Memoranda and Legal Investment
Surveys (including related fees and expenses of counsel to the Underwriters);
(g) all fees and expenses of the registrar and transfer agent of the Stock; and
(h) all other costs and expenses incident to the


                                       11


<PAGE>


performance of the obligations of the Company under this Agreement (including,
without limitation, the fees and expenses of the Company's counsel and the
Company's independent accountants); PROVIDED that, except as otherwise provided
in this Section 5 and in Section 10, the Underwriters shall pay their own costs
and expenses, including the fees and expenses of their counsel, any transfer
taxes on the Stock which they may sell and the expenses of advertising any
offering of the Stock made by the Underwriters.

6. CONDITIONS OF UNDERWRITERS' OBLIGATIONS. The respective obligations of the
several Underwriters hereunder are subject to the accuracy, when made and on
each of the Closing Dates, of the representations and warranties of the Company
contained herein, to the accuracy of the statements of the Company made in any
certificates pursuant to the provisions hereof, to the performance by the
Company of their obligations hereunder, and to each of the following additional
terms and conditions:

     (a) No stop order suspending the effectiveness of either of the
     Registration Statements shall have been issued and no proceedings for that
     purpose shall have been initiated or threatened by the Commission, and any
     request for additional information on the part of the Commission (to be
     included in the Registration Statements or the Prospectus or otherwise)
     shall have been complied with to the reasonable satisfaction of the
     Representatives. The Rule 462(b) Registration Statement, if any, and the
     Prospectus shall have been timely filed with the Commission in accordance
     with Section 4(a).

     (b) None of the Underwriters shall have discovered from and after the date
     hereof and disclosed to the Company on or prior to the Closing Date that
     the Registration Statements or the Prospectus or any amendment or
     supplement thereto contains an untrue statement of a fact which, in the
     opinion of counsel for the Underwriters, is material or omits to state any
     fact which, in the opinion of such counsel, is material and is required to
     be stated therein or is necessary to make the statements therein not
     misleading.

     (c) All corporate proceedings and other legal matters incident to the
     authorization, form and validity of each of this Agreement, the Stock, the
     Registration Statements and the Prospectus and all other legal matters
     relating to this Agreement and the transactions contemplated hereby shall
     be reasonably satisfactory in all material respects to counsel for the
     Underwriters, and the Company shall have furnished to such counsel all
     documents and information that they may reasonably request to enable them
     to pass upon such matters.

     (d) Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. shall have
     furnished to the Representatives such counsel's written opinion, as counsel
     to the Company, addressed to the Underwriters and dated the Closing Date,
     in form and substance reasonably satisfactory to the Representatives, to
     the effect that:

          (i)  The Company and each of its subsidiaries have been duly
               incorporated and are validly existing as corporations in good
               standing under the laws of their respective jurisdictions of
               incorporation and have all power and authority necessary to own
               or hold their respective properties and to conduct the businesses
               in which they are engaged, except where the failure to have such
               power or authority would not have, singularly or in the
               aggregate, a Material Adverse Effect.

          (ii) The Company has an authorized capitalization as set forth in the
               Prospectus as of the date stated therein, and all of the issued
               shares of capital stock of the Company, including the Stock being
               delivered on the Closing Date, upon issuance and payment
               therefore in accordance with this Agreement have been duly and
               validly authorized and issued, are fully paid and non-assessable
               and conform to the description thereof contained in the
               Prospectus.


                                       12


<PAGE>


          (iii) All the outstanding shares of capital stock of each subsidiary
               of the Company have been duly authorized and validly issued, are
               fully paid and nonassessable and, except to the extent set forth
               in the Prospectus, are owned by the Company directly or
               indirectly through one or more wholly-owned subsidiaries, free
               and clear of any claim, lien, encumbrance, security interest,
               restriction upon voting or transfer or any other claim of any
               third party.

          (iv) To such counsel's knowledge, there are no preemptive or other
               rights to subscribe for or to purchase, nor any restriction upon
               the voting or transfer of, any shares of the Stock pursuant to
               the Company's charter or by-laws or any agreement.

          (v)  This Agreement has been duly authorized, executed and delivered
               by the Company.

          (vi) The execution, delivery and performance of this Agreement and the
               consummation of the transactions contemplated hereby will not, to
               our knowledge, conflict with or result in a breach or violation
               of any of the terms or provisions of, or constitute a default
               under any indenture, mortgage, deed of trust, loan agreement or
               other material agreement or instrument known to such counsel
               after reasonable investigation to which the Company or any of its
               subsidiaries is a party or by which the Company or any of its
               subsidiaries is bound or to which any of the properties or assets
               of the Company or any of its subsidiaries is subject, which is
               filed as an exhibit to the Registration Statement or incorporated
               by reference therein, nor will such actions result in any
               violation of the Charter or by-laws of the Company or of any of
               its subsidiaries or, to our knowledge, any statute or any order,
               rule or regulation of any court or governmental agency or body or
               court having jurisdiction over the Company or any of its
               subsidiaries or any of their properties or assets.

          (vii) Except for applicable state and foreign securities laws (as to
               which we express no opinion) and the registration of the Stock
               under the Securities Act in connection with the purchase and
               distribution of the Stock by the Underwriters, no consent,
               approval, authorization or order of, or filing or registration
               with, any such court or governmental agency or body is required
               on the part of the Company for the execution and delivery of this
               Agreement by the Company and the consummation by the Company of
               the transactions contemplated hereby.

          (viii) The statements in (A) the Prospectus under the heading
               "Description of Capital Stock" and (B) in the Registration
               Statements in Item 15-Indemnification of Directors and Officers,
               in each case insofar as such statements constitute summaries of
               the legal matters, documents or proceedings referred to therein,
               have been reviewed by such counsel and fairly summarize the
               matters described therein in all material respects.

          (ix) To the best of such counsel's knowledge, there are no legal or
               governmental proceedings, contracts or other documents of a
               character required to be described in the Registration Statements
               or Prospectus or to be filed as exhibits to the Registration
               Statements which are not described or filed as required.


                                       13


<PAGE>


          (x)  To the best of such counsel's knowledge, neither the Company nor
               any of its subsidiaries (i) is in violation of its charter or
               by-laws or (ii) is in default, and no event has occurred, which,
               with notice or lapse of time or both, would constitute a default,
               in the due performance or observance of any material agreement to
               which the Company is a party or by which it is bound or to which
               any of its properties or assets is subject, which is filed as an
               exhibit to the Registration Statement or incorporated by
               reference therein, except for those defaults which, either
               individually or in the aggregate, would not have a Material
               Adverse Effect.

          (xi) To the best of such counsel's knowledge and other than as set
               forth in the Prospectus, there are no legal or governmental
               proceedings pending to which the Company or any of its
               subsidiaries is a party or of which any property or asset of the
               Company or any of its subsidiaries is the subject which,
               singularly or in the aggregate, if determined adversely to the
               Company or any of its subsidiaries, is reasonably expected to
               have a Material Adverse Effect or would prevent or adversely
               affect the ability of the Company to perform its obligations
               under this Agreement; and, to the best of such counsel's
               knowledge, no such proceedings are threatened or contemplated by
               governmental authorities or threatened by others.

          (xii) The Initial Registration Statement was declared effective under
               the Securities Act as of the date and time specified in such
               opinion and no stop order suspending the effectiveness of the
               Registration Statements has been issued and, to the knowledge of
               such counsel, no proceeding for that purpose is pending or
               threatened by the Commission.

          (xiii) The Registration Statements, as of the respective effective
               dates and the Prospectus, as of its date, and any further
               amendments or supplements thereto, as of their respective dates,
               made by the Company prior to the Closing Date (other than the
               financial statements and related schedules and other financial
               and statistical data contained therein, as to which such counsel
               need express no opinion) complied as to form in all material
               respects with the requirements of the Securities Act and the
               Rules and Regulations; and the documents incorporated by
               reference in the Prospectus and any further amendment or
               supplement to any such incorporated document made by the Company
               prior to the Closing Date (other than the financial statements
               and related schedules and other financial data contained therein,
               as to which such counsel need express no opinion), when they
               became effective or were filed with the Commission, as the case
               may be, complied as to form in all material respects with the
               requirements of the Securities Act or the Exchange Act, as
               applicable, and the rules and regulations of the Commission
               thereunder.

          (xiv) To the best of such counsel's knowledge, no person or entity has
               the right to require registration of shares of Common Stock or
               other securities of the Company because of the filing or
               effectiveness of the Registration Statements or otherwise, except
               for persons and entities who have expressly waived such right or
               who have been given proper notice and have failed to exercise
               such right within the time or times required under the terms and
               conditions of such right or have been given an "underwriters'
               cut-back" notice in accordance with the terms of the applicable
               agreement under which such registration rights arose.


                                       14


<PAGE>


          (xv) Neither the Company nor any of its subsidiaries is or, after
               giving effect to the offering and sale of the Stock and the
               application of the proceeds thereof as described in the
               Prospectus, will be an "investment company" within the meaning of
               the Investment Company Act and the rules and regulations of the
               Commission thereunder.

               Such counsel shall also have furnished to the Representatives a
               written statement, addressed to the Underwriters and dated the
               Closing Date, in form and substance satisfactory to the
               Representatives, to the effect that (x) such counsel has acted as
               counsel to the Company in connection with the preparation of the
               Registration Statements (y) based on such counsel's examination
               of the Registration Statements and such counsel's investigations
               made in connection with the preparation of the Registration
               Statements and conferences with certain officers and employees of
               and with auditors for and counsel to the Company, and although
               such counsel is not passing upon and does not assume any
               responsibility for the accuracy, completeness or fairness of the
               statements contained in the Registration Statements, nothing has
               come to the attention of such counsel that would lead them to
               believe that (I) the Registration Statements, as of the
               respective effective dates, contained any untrue statement of a
               material fact or omitted to state any material fact required to
               be stated therein or necessary in order to make the statements
               therein not misleading, or that the Prospectus contains any
               untrue statement of a material fact or omits to state any
               material fact required to be stated therein or necessary in order
               to make the statements therein, in light of the circumstances
               under which they were made, not misleading; it being understood
               that such counsel need express no opinion as to the financial
               statements or other financial or statistical data contained in
               the Registration Statement or the Prospectus.

               The foregoing opinion and statement may be qualified by a
     statement to the effect that such counsel has not independently verified
     the accuracy, completeness or fairness of the statements contained in the
     Registration Statement or the Prospectus and takes no responsibility
     therefor except to the extent set forth in the opinion described in clauses
     (viii) and (ix) above.

     (e) Sughrue, Mion, Zinn, Macpeak & Seas, PLLC shall have furnished to the
     Representatives such counsel's written opinion, as counsel to the Company,
     addressed to the Underwriters and dated the Closing Date, in form and
     substance reasonably satisfactory to the Representatives, to the effect
     that:

          (i)  The statements set forth under the headings "Risk Factors--If we
               are unable to protect our intellectual property rights
               adequately, the value of our TAP technology and our product
               candidates could be diminished.", "Risk Factors--We may be
               subject to substantial costs and liability or be prohibited from
               commercializing our potential products as a result of litigation
               and other proceedings relating to patent rights." and
               "Business--Patents and Proprietary Technology" in the Prospectus
               constitute an accurate summary of the matters described therein.

          (ii) Such counsel has no reason to believe that the information under
               the headings "Risk Factors--If we are unable to protect our
               intellectual property rights adequately, the value of our TAP
               technology and our product candidates could be diminished.",
               "Risk Factors--We may be subject to substantial costs and
               liability or be prohibited from


                                       15


<PAGE>


               commercializing our potential products as a result of litigation
               and other proceedings relating to patent rights." and
               "Business--Patents and Proprietary Technology" in the
               Registration Statement, as of its effective date, contained any
               untrue statement of a material fact or omitted to state any
               material fact with respect to patents and trade secrets required
               to be stated therein or necessary to make the statements therein
               not misleading or that the statements set forth under the
               headings "Risk Factors--If we are unable to protect our
               intellectual property rights adequately, the value of our TAP
               technology and our product candidates could be diminished.",
               "Risk Factors--We may be subject to substantial costs and
               liability or be prohibited from commercializing our potential
               products as a result of litigation and other proceedings relating
               to patent rights." and "Business--Patents and Proprietary
               Technology" in the Prospectus, as of the date of this opinion,
               included or includes any untrue statement of a material fact or
               omitted or omits to state a material fact with respect to patents
               and trade secrets necessary in order to make the statements
               therein, in the light of the circumstances under which they were
               made, not misleading.

     (f) The Representatives shall have received from Shearman & Sterling
     counsel for the Underwriters, such opinion or opinions, dated the Closing
     Date, with respect to such matters as the Underwriters may reasonably
     require, and the Company shall have furnished to such counsel such
     documents as they request for enabling them to pass upon such matters.

     (g) At the time of the execution of this Agreement, the Representatives
     shall have received from PricewaterhouseCoopers LLP a letter, addressed to
     the Underwriters and dated such date, in form and substance satisfactory to
     the Representatives (i) confirming that they are independent certified
     public accountants with respect to the Company and its subsidiaries within
     the meaning of the Securities Act and the Rules and Regulations and (ii)
     stating the conclusions and findings of such firm with respect to the
     financial statements and certain financial information contained or
     incorporated by reference in the Prospectus.

     (h) On the Closing Date, the Representatives shall have received a letter
     (the "bring-down letter") from PricewaterhouseCoopers addressed to the
     Underwriters and dated the Closing Date confirming, as of the date of the
     bring-down letter (or, with respect to matters involving changes or
     developments since the respective dates as of which specified financial
     information is given in the Prospectus as of a date not more than three
     business days prior to the date of the bring-down letter), the conclusions
     and findings of such firm with respect to the financial information and
     other matters covered by its letter delivered to the Representatives
     concurrently with the execution of this Agreement pursuant to Section 6(g).

     (i) The Company shall have furnished to the Representatives a certificate,
     dated the Closing Date, and executed on its behalf by its Chairman of the
     Board, its President or a Vice President and its chief financial officer
     stating that (i) such officers have carefully examined the Registration
     Statements and the Prospectus and the Registration Statements as of their
     respective effective dates and the Prospectus, as of the date it was filed
     with the Commission and as of the dater hereof, did not and does not
     include any untrue statement of a material fact and did not omit to state a
     material fact required to be stated therein or necessary to make the
     statements therein not misleading, (ii) since the effective date of the
     Initial Registration Statement no event has occurred which should have been
     set forth in a supplement or amendment to the Registration Statements or
     the Prospectus, (iii) as of the Closing Date, the representations and
     warranties of the Company in this Agreement are true and correct and the
     Company has complied in all material respects with all agreements and
     satisfied in all material respects all conditions on its


                                       16


<PAGE>


     part to be performed or satisfied hereunder at or prior to the Closing
     Date, and (iv) subsequent to the date of the most recent financial
     statements included or incorporated by reference in the Prospectus, there
     has been no material adverse change in the financial position or results of
     operation of the Company and its subsidiaries, or any change, or any
     development including a prospective change, in or affecting the condition
     (financial or otherwise), results of operations, business or prospects of
     the Company and its subsidiaries taken as a whole, except as set forth in
     the Prospectus.

     (j) Neither the Company nor any of its subsidiaries shall have sustained
     since the date of the latest audited financial statements included or
     incorporated by reference in the Prospectus any loss or interference with
     its business from fire, explosion, flood or other calamity, whether or not
     covered by insurance, or from any labor dispute or court or governmental
     action, order or decree, otherwise than as set forth or contemplated in the
     Prospectus (ii) since such date there shall not have been any change in the
     capital stock or long-term debt of the Company or any of its subsidiaries
     or any change, or any development involving a prospective change, in or
     affecting the business, general affairs, management, financial position,
     stockholders' equity or results of operations of the Company and its
     subsidiaries, otherwise than as set forth or contemplated in the
     Prospectus, the effect of which, in any such case described in clause (i)
     or (ii), is, in the judgment of the Representatives, so material and
     adverse as to make it impracticable or inadvisable to proceed with the sale
     or delivery of the Stock on the terms and in the manner contemplated in the
     Prospectus.

     (k) No action shall have been taken and no statute, rule, regulation or
     order shall have been enacted, adopted or issued by any governmental agency
     or body which would, as of the Closing Date, prevent the issuance or sale
     of the Stock; and no injunction, restraining order or order of any other
     nature by any federal or state court of competent jurisdiction shall have
     been issued as of the Closing Date which would prevent the issuance or sale
     of the Stock.

     (l) Subsequent to the execution and delivery of this Agreement there shall
     not have occurred any of the following: (i) trading in securities generally
     on the New York Stock Exchange or the American Stock Exchange or in the
     over-the-counter market, or trading in any securities of the Company on any
     exchange or in the over-the-counter market, shall have been suspended or
     minimum prices shall have been established on any such exchange or such
     market by the Commission, by such exchange or by any other regulatory body
     or governmental authority having jurisdiction, (ii) a banking moratorium
     shall have been declared by Federal or state authorities, (iii) the United
     States shall have become engaged in hostilities, there shall have been an
     escalation in hostilities involving the United States or there shall have
     been a declaration of a national emergency or war by the United States or
     (iv) there shall have occurred such a material adverse change in general
     economic, political or financial conditions (or the effect of international
     conditions on the financial markets in the United States shall be such) as
     to make it, in the judgment of the Representatives, impracticable or
     inadvisable to proceed with the sale or delivery of the Stock on the terms
     and in the manner contemplated in the Prospectus.

     (m) The Company shall have filed a supplemental listing application
     relating to the Stock with the Nasdaq National Market System.

     (n) SG Cowen shall have received the written agreements, substantially in
     the form of Exhibit I hereto, of the officers, directors and shareholders
     of the Company listed in Schedule C to this Agreement.

All opinions, letters, evidence and certificates mentioned above or elsewhere in
this Agreement shall be deemed to be in compliance with the provisions hereof
only if they are in form and substance reasonably satisfactory to counsel for
the Underwriters.


                                       17


<PAGE>


7.   INDEMNIFICATION AND CONTRIBUTION.

     (a) The Company shall indemnify and hold harmless each Underwriter, its
     officers, employees, representatives and agents and each person, if any,
     who controls any Underwriter within the meaning of the Securities Act
     (collectively the "Underwriter Indemnified Parties" and, each an
     "Underwriter Indemnified Party") against any loss, claim, damage or
     liability, joint or several, or any action in respect thereof, to which
     that Underwriter Indemnified Party may become subject, under the Securities
     Act or otherwise, insofar as such loss, claim, damage, liability or action
     arises out of or is based upon (i) any untrue statement or alleged untrue
     statement of a material fact contained in the Preliminary Prospectus,
     either of the Registration Statements or the Prospectus or in any amendment
     or supplement thereto, (ii) the omission or alleged omission to state in
     any Preliminary Prospectus, either of the Registration Statements or the
     Prospectus or in any amendment or supplement thereto a material fact
     required to be stated therein or necessary to make the statements therein
     not misleading and shall reimburse each Underwriter Indemnified Party
     promptly upon demand for any legal or other expenses reasonably incurred by
     that Underwriter Indemnified Party in connection with investigating or
     preparing to defend or defending against or appearing as a third party
     witness in connection with any such loss, claim, damage, liability or
     action as such expenses are incurred; PROVIDED, HOWEVER, (i) in the event
     that it is finally judicially determined that the Underwriters were not
     entitled to receive payments for legal and other expenses pursuant to this
     subparagraph, the Underwriters will promptly return all sums that had been
     advanced pursuant hereto; (ii) that the Company shall not be liable in any
     such case to the extent that any such loss, claim, damage, liability or
     action arises out of or is based upon an untrue statement or alleged untrue
     statement in or omission or alleged omission from the Preliminary
     Prospectus, either of the Registration Statements or the Prospectus or any
     such amendment or supplement in reliance upon and in conformity with
     written information furnished to the Company through the Representatives by
     or on behalf of any Underwriter specifically for use therein, which
     information the parties hereto agree is limited to the Underwriter's
     Information (as defined in Section 16); and (iii) the foregoing
     indemnification agreement with respect to the Preliminary Prospectus shall
     not inure to the benefit of any Underwriter from whom the person asserting
     any such loss, claim, damage or liability purchased Stock, or any officers,
     employees, representatives, agents or controlling persons of such
     Underwriter, if (A) a copy of the Prospectus (as then amended or
     supplemented) was required by law to be delivered to such person at or
     prior to the written confirmation of the sale of Stock to such person, (B)
     a copy of the Prospectus (as then amended or supplemented) excluding
     documents incorporated by reference therein was not sent or given to such
     person by or on behalf of such Underwriter and such failure was not due to
     non-compliance by the Company with Section 4(d), and (C) the Prospectus (as
     so amended or supplemented) would have cured the defect giving rise to such
     loss, claim, damage or liability. This indemnity agreement is not exclusive
     and will be in addition to any liability which the Company might otherwise
     have and shall not limit any rights or remedies which may otherwise be
     available at law or in equity to each Underwriter Indemnified Party.

     (b) Each Underwriter, severally and not jointly, shall indemnify and hold
     harmless the Company its officers, employees, representatives and agents,
     each of its directors and each person, if any, who controls the Company
     within the meaning of the Securities Act (collectively the "Company
     Indemnified Parties" and each a "Company Indemnified Party") against any
     loss, claim, damage or liability, joint or several, or any action in
     respect thereof, to which the Company Indemnified Parties may become
     subject, under the Securities Act or otherwise, insofar as such loss,
     claim, damage, liability or action arises out of or is based upon (i) any
     untrue statement or alleged untrue statement of a material fact contained
     in the Preliminary Prospectus, either of the Registration Statements or the
     Prospectus or in any amendment or supplement thereto or (ii) the omission
     or alleged omission to state therein a material fact required to be stated
     therein or


                                       18


<PAGE>


     necessary to make the statements therein not misleading, but in each case
     only to the extent that the untrue statement or alleged untrue statement or
     omission or alleged omission was made in reliance upon and in conformity
     with written information furnished to the Company through the
     Representatives by or on behalf of that Underwriter specifically for use
     therein, and shall reimburse the Company Indemnified Parties for any legal
     or other expenses reasonably incurred by such parties in connection with
     investigating or preparing to defend or defending against or appearing as
     third party witness in connection with any such loss, claim, damage,
     liability or action as such expenses are incurred; provided that the
     parties hereto hereby agree that such written information provided by the
     Underwriters consists solely of the Underwriter's Information. This
     indemnity agreement is not exclusive and will be in addition to any
     liability which the Underwriters might otherwise have and shall not limit
     any rights or remedies which may otherwise be available at law or in equity
     to the Company Indemnified Parties.

     (c) Promptly after receipt by an indemnified party under this Section 7 of
     notice of any claim or the commencement of any action, the indemnified
     party shall, if a claim in respect thereof is to be made against the
     indemnifying party under this Section 7, notify the indemnifying party in
     writing of the claim or the commencement of that action; PROVIDED, HOWEVER,
     that the failure to notify the indemnifying party shall not relieve it from
     any liability which it may have under this Section 7 except to the extent
     it has been materially prejudiced by such failure; and, PROVIDED, FURTHER,
     that the failure to notify the indemnifying party shall not relieve it from
     any liability which it may have to an indemnified party otherwise than
     under this Section 7. If any such claim or action shall be brought against
     an indemnified party, and it shall notify the indemnifying party thereof,
     the indemnifying party shall be entitled to participate therein and, to the
     extent that it wishes, jointly with any other similarly notified
     indemnifying party, to assume the defense thereof with counsel reasonably
     satisfactory to the indemnified party. After notice from the indemnifying
     party to the indemnified party of its election to assume the defense of
     such claim or action, the indemnifying party shall not be liable to the
     indemnified party under this Section 7 for any legal or other expenses
     subsequently incurred by the indemnified party in connection with the
     defense thereof other than reasonable costs of investigation; PROVIDED,
     HOWEVER, that any indemnified party shall have the right to employ separate
     counsel in any such action and to participate in the defense thereof but
     the fees and expenses of such counsel shall be at the expense of such
     indemnified party unless (i) the employment thereof has been specifically
     authorized by the indemnifying party in writing, (ii) such indemnified
     party shall have been advised by such counsel that there may be one or more
     legal defenses available to it which are different from or additional to
     those available to the indemnifying party and in the reasonable judgment of
     such counsel it is advisable for such indemnified party to employ separate
     counsel or (iii) the indemnifying party has failed to assume the defense of
     such action and employ counsel reasonably satisfactory to the indemnified
     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 of such action on behalf of such indemnified party,
     it being understood, however, that the indemnifying party shall not, in
     connection with any one such action or separate but substantially similar
     or related actions in the same jurisdiction arising out of the same general
     allegations or circumstances, be liable for the reasonable fees and
     expenses of more than one separate firm of attorneys at any time for all
     such indemnified parties, which firm shall be designated in writing by SG
     Cowen if the indemnified parties under this Section 7 consist of any
     Underwriter Indemnified Party, or by the Company if the indemnified parties
     under this Section 7 consist of any Company Indemnified Parties. Each
     indemnified party, as a condition of the indemnity agreements contained in
     Sections 7(a) and 7(b), shall use all reasonable efforts to cooperate with
     the indemnifying party in the defense of any such action or claim. Subject
     to the provisions of Section 7(d) below, no indemnifying party shall be
     liable for any settlement of any such action effected without its written
     consent (which consent shall not be unreasonably withheld), but if settled
     with its written consent or if there be a final judgment for the plaintiff
     in any such action,


                                       19


<PAGE>


     the indemnifying party agrees to indemnify and hold harmless any
     indemnified party from and against any loss or liability by reason of such
     settlement or judgment.

     (d) If at any time an indemnified party shall have requested that an
     indemnifying party reimburse the indemnified party for fees and expenses of
     counsel, such indemnifying party agrees that it shall be liable for any
     settlement of the nature contemplated by this Section 7 effected without
     its written consent if (i) such settlement is entered into more than 45
     days after receipt by such indemnifying party of the request for
     reimbursement, (ii) such indemnifying party shall have received notice of
     the terms of such settlement at least 30 days prior to such settlement
     being entered into and (iii) such indemnifying party shall not have
     reimbursed such indemnified party in accordance with such request prior to
     the date of such settlement.

     (e) If the indemnification provided for in this Section 7 is unavailable or
     insufficient to hold harmless an indemnified party under Section 7(a) or
     7(b), then each indemnifying party shall, in lieu of indemnifying such
     indemnified party, contribute to the amount paid or payable by such
     indemnified party as a result of such loss, claim, damage or liability, or
     action in respect thereof, (i) in such proportion as shall be appropriate
     to reflect the relative benefits received by the Company on the one hand
     and the Underwriters on the other from the offering of the Stock or if the
     allocation provided by clause (i) above is not permitted by applicable law,
     in such proportion as is appropriate to reflect not only the relative
     benefits referred to in clause (i) above but also the relative fault of the
     Company on the one hand and the Underwriters on the other with respect to
     the statements or omissions which resulted in such loss, claim, damage or
     liability, or action in respect thereof, as well as any other relevant
     equitable considerations. The relative benefits received by the Company on
     the one hand and the Underwriters on the other with respect to such
     offering shall be deemed to be in the same proportion as the total net
     proceeds from the offering of the Stock purchased under this Agreement
     (before deducting expenses) received by the Company bear to the total
     underwriting discounts and commissions received by the Underwriters with
     respect to the Stock purchased under this Agreement, in each case as set
     forth in the table on the cover page of the Prospectus. The relative fault
     shall be determined by reference to, among other things, whether the untrue
     or alleged untrue statement of a material fact or the omission or alleged
     omission to state a material fact relates to information supplied by the
     Company on the one hand or the Underwriters on the other, the intent of the
     parties and their relative knowledge, access to information and opportunity
     to correct or prevent such untrue statement or omission; provided that the
     parties hereto agree that the written information furnished to the Company
     through the Representatives by or on behalf of the Underwriters for use in
     any Preliminary Prospectus, either of the Registration Statements or the
     Prospectus consists solely of the Underwriter's Information. The Company
     and the Underwriters agree that it would not be just and equitable if
     contributions pursuant to this Section 7(e) were to be determined by pro
     rata allocation (even if the Underwriters were treated as one entity for
     such purpose) or by any other method of allocation which does not take into
     account the equitable considerations referred to herein. The amount paid or
     payable by an indemnified party as a result of the loss, claim, damage or
     liability, or action in respect thereof, referred to above in this Section
     7(e) shall be deemed to include, for purposes of this Section 7(e), any
     legal or other expenses reasonably incurred by such indemnified party in
     connection with investigating or defending any such action or claim.
     Notwithstanding the provisions of this Section 7(e), no Underwriter shall
     be required to contribute any amount in excess of the amount by which the
     total price at which the Stock underwritten by it and distributed to the
     public were offered to the public less the amount of any damages which such
     Underwriter has otherwise paid or become liable to pay by reason of any
     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.


                                       20


<PAGE>


The Underwriters' obligations to contribute as provided in this Section 7(e) are
several in proportion to their respective underwriting obligations and not
joint.

8. TERMINATION. The obligations of the Underwriters hereunder may be terminated
by SG Cowen, in its absolute discretion by notice given to and received by the
Company prior to delivery of and payment for the Firm Stock if, prior to that
time, any of the events described in Sections 6(j) or 6(l) have occurred or if
the Underwriters shall decline to purchase the Stock for any reason permitted
under this Agreement.

9. REIMBURSEMENT OF UNDERWRITERS' EXPENSES. If (a) this Agreement shall have
been terminated pursuant to Section 8 or 10 (except as provided below), (b) the
Company shall fail to tender the Stock for delivery to the Underwriters for any
reason permitted under this Agreement, or (c) the Underwriters shall decline to
purchase the Stock for any reason permitted under this Agreement the Company
shall reimburse the Underwriters for the fees and expenses of their counsel and
for such other out-of-pocket expenses as shall have been reasonably incurred by
them in connection with this Agreement and the proposed purchase of the Stock,
and upon demand the Company shall pay the full amount thereof to the SG Cowen;
provided, however, that the Company shall not be liable to any of the several
Underwriters for damages on account of loss of anticipated profits from the sale
by them of the Stock. If this Agreement is terminated pursuant to Section 10 by
reason of the default of one or more Underwriters, the Company shall not be
obligated to reimburse any defaulting Underwriter on account of those expenses.

10. SUBSTITUTION OF UNDERWRITERS. If any Underwriter or Underwriters shall
default in its or their obligations to purchase shares of Stock hereunder and
the aggregate number of shares which such defaulting Underwriter or Underwriters
agreed but failed to purchase does not exceed ten percent (10%) of the total
number of shares underwritten, the other Underwriters shall be obligated
severally, in proportion to their respective commitments hereunder, to purchase
the shares which such defaulting Underwriter or Underwriters agreed but failed
to purchase. If any Underwriter or Underwriters shall so default and the
aggregate number of shares with respect to which such default or defaults occur
is more than ten percent (10%) of the total number of shares underwritten and
arrangements satisfactory to the Representatives and the Company for the
purchase of such shares by other persons are not made within forty-eight (48)
hours after such default, this Agreement shall terminate.

     If the remaining Underwriters or substituted Underwriters are required
hereby or agree to take up all or part of the shares of Stock of a defaulting
Underwriter or Underwriters as provided in this Section 10, (i) the Company
shall have the right to postpone the applicable Closing Date for a period of not
more than five (5) full business days in order that the Company may effect
whatever changes may thereby be made necessary in the Registration Statements or
the Prospectus, or in any other documents or arrangements, and the Company
agrees promptly to file any amendments to the Registration Statement or
supplements to the Prospectus which may thereby be made necessary, and (ii) the
respective numbers of shares to be purchased by the remaining Underwriters or
substituted Underwriters shall be taken as the basis of their underwriting
obligation for all purposes of this Agreement. Nothing herein contained shall
relieve any defaulting Underwriter of its liability to the Company or the other
Underwriters for damages occasioned by its default hereunder. Any termination of
this Agreement pursuant to this Section 10 shall be without liability on the
part of any non-defaulting Underwriter or the Company, except expenses to be
paid or reimbursed pursuant to Sections 5 and 9 and except the provisions of
Section 7 shall not terminate and shall remain in effect.

11. SUCCESSORS; PERSONS ENTITLED TO BENEFIT OF AGREEMENT. This Agreement shall
inure to the benefit of and be binding upon the several Underwriters, the
Company and their respective successors. Nothing expressed or mentioned in this
Agreement is intended or shall be construed to give any person other than the
persons mentioned in the preceding sentence any legal or equitable right, remedy
or claim under or in respect of this Agreement, or any provisions herein
contained, this


                                       21


<PAGE>


Agreement and all conditions and provisions hereof being intended to be and
being for the sole and exclusive benefit of such persons and for the benefit of
no other person; except that the representations, warranties, covenants,
agreements and indemnities of the Company contained in this Agreement shall also
be for the benefit of the Underwriter Indemnified Parties, and the indemnities
of the several Underwriters shall also be for the benefit of the Company
Indemnified Parties.

12. SURVIVAL OF INDEMNITIES, REPRESENTATIONS, WARRANTIES, ETC. The respective
indemnities, covenants, agreements, representations, warranties and other
statements of the Company and the several Underwriters, as set forth in this
Agreement or made by them respectively, pursuant to this Agreement, shall remain
in full force and effect, regardless of any investigation made by or on behalf
of any Underwriter, the Company or any person controlling any of them and shall
survive delivery of and payment for the Stock.

13. NOTICES. All statements, requests, notices and agreements hereunder shall be
in writing, and:

     (a) if to the Underwriters, shall be delivered or sent by mail, telex or
     facsimile transmission to SG Securities Corporation Attention: [         ]
     (Fax: 212-[        ]); or

     (b) if to the Company shall be delivered or sent by mail, telex or
     facsimile transmission to ImmunoGen, Inc., 128 Sidney Street, Cambridge, MA
     02139, Attention: Mitchel Sayare, President and Chief Executive Officer
     (Fax: 617-995-2510), with a copy to William T. Whelan, Esq., Mintz, Levin,
     Cohn, Ferris, Glovsky and Popeo, P.C., One Financial Center, Boston, MA
     02111 (Fax: 617-542-2241).

14. DEFINITION OF CERTAIN TERMS. For purposes of this Agreement, (a) "business
day" means any day on which the New York Stock Exchange, Inc. is open for
trading and (b) "subsidiary" has the meaning set forth in Rule 405 of the Rules
and Regulations.

15. GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

16. UNDERWRITERS' INFORMATION. The parties hereto acknowledge and agree that,
for all purposes of this Agreement, the Underwriters' Information consists
solely of the following information in the Prospectus: (i) the last paragraph on
the front cover page concerning the terms of the offering by the Underwriters;
and (ii) the statements concerning the Underwriters contained in the [INSERT
REFERENCES TO APPROPRIATE PARAGRAPHS] under the heading "Underwriting."

17. AUTHORITY OF THE REPRESENTATIVES. In connection with this Agreement, you
will act for and on behalf of the several Underwriters, and any action taken
under this Agreement by the Representatives, will be binding on all the
Underwriters.

18. PARTIAL UNENFORCEABILITY. The invalidity or unenforceability of any Section,
paragraph or provision of this Agreement shall not affect the validity or
enforceability of any other Section, paragraph or provision hereof. If any
Section, paragraph or provision of this Agreement is for any reason determined
to be invalid or unenforceable, there shall be deemed to be made such minor
changes (and only such minor changes) as are necessary to make it valid and
enforceable.

19. GENERAL. This Agreement constitutes the entire agreement of the parties to
this Agreement and supersedes all prior written or oral and all contemporaneous
oral agreements, understandings and negotiations with respect to the subject
matter hereof. In this Agreement, the masculine, feminine and neuter genders and
the singular and the plural include one another. The section headings in this
Agreement are for the convenience of the parties only and will not affect the
construction


                                       22


<PAGE>


or interpretation of this Agreement. This Agreement may be amended or modified,
and the observance of any term of this Agreement may be waived, only by a
writing signed by the Company and the Representatives.

20. COUNTERPARTS. This Agreement may be signed in any number of counterparts,
each of which shall be an original, with the same effect as if the signatures
thereto and hereto were upon the same instrument.






                                       23


<PAGE>


         If the foregoing is in accordance with your understanding of the
agreement between the Company and the several Underwriters, kindly indicate your
acceptance in the space provided for that purpose below.

                                   Very truly yours,

                                   IMMUNOGEN, INC.


                                   By:____________________________
                                         Name:
                                         Title:


Accepted as of
the date first above written:

SG COWEN SECURITIES CORPORATION
FLEETBOSTON ROBERTSON STEPHENS, INC.
ADAMS HARKNESS & HILL, INC.
Acting on their own behalf
and as Representatives of several
Underwriters referred to in the
foregoing Agreement.

By:  SG COWEN SECURITIES CORPORATION



By:______________________________
      Name:
      Title:



                                       24


<PAGE>


                                   SCHEDULE A


<TABLE>
<CAPTION>
                                                  Number           Number of
                                                  of Firm          Optional
                                                  Shares            Shares
                                                   to be             to be
Name                                             Purchased         Purchased
----                                             ---------         ---------
<S>                                              <C>               <C>
SG Cowen Securities Corporation

Robertson Stephens, Inc.

Adams Harkness & Hill, Inc.



                                                 ---------         ---------

Total
                                                 =========         =========
</TABLE>





                                       25


<PAGE>


                                   SCHEDULE B

                 [list of shareholders subject to Section 4(h)]




                                       26


<PAGE>


                                                                       Exhibit I


                           [Form of Lock-Up Agreement]


                                                               ___________, 2000

SG Cowen Securities Corporation
Robertson Stephens, Inc.
Adams Harkness & Hill, Inc.
  As representatives of the
  several Underwriters
  c/o SG Cowen Securities Corporation
Financial Square
New York, New York  10005

                               Re: ImmunoGen, Inc.

Dear Sirs:

     In order to induce SG Cowen Securities Corporation ("SG Cowen") and
Robertson Stephens, Inc., Adams Harkness & Hill, Inc. (together with SG Cowen,
the "Representatives"), to enter in to a certain underwriting agreement with
ImmunoGen, Inc., a Massachusetts corporation (the "Company"), with respect to
the public offering of shares of the Company's Common Stock, par value $.01 per
share ("Common Stock"), the undersigned hereby agrees that during the period
commencing on the date hereof and ending 90 days following the date of the final
prospectus relating to such public offering, the undersigned will not, without
the prior written consent of SG Cowen, directly or indirectly, offer, sell,
assign, transfer, pledge, contract to sell, grant any option, right or warrant
to purchase, lend or otherwise transfer or dispose of, or enter into any swap
hedge or other arrangement that transfers to another (in whole or in part) any
of the economic consequences of ownership of, any shares of Common Stock
(including, without limitation, Common Stock which may be deemed to be
beneficially owned by the undersigned in accordance with the rules and
regulations promulgated under the Securities Act of 1933, as the same may be
amended or supplemented from time to time (such shares, the "Beneficially Owned
Shares") or securities convertible into or exercisable or exchangeable in Common
Stock.

     The foregoing sentence shall not apply to (a) the transfer of any shares of
Common Stock pursuant to a bona fide gift or gifts, (b) the transfer, if the
undersigned is an individual, of any shares of Common Stock to his or her
immediate family or a trust, the beneficiaries of which are exclusively the
undersigned and/or a member or members of his or her immediate family, either
during his or her lifetime or on death by will or intestacy, (c) the transfer of
any shares of Common Stock to a charitable organization, (d) the transfer, if
the undersigned is a partnership or a corporation, to limited partners or
shareholders of the undersigned as a distribution, or (e) the transfer of any
shares of securities to any company, corporation, business or entity controlled
by, controlling, or under common control with the undersigned. For this purpose,
"control" means direct or indirect beneficial ownership of one hundred percent
(100%) interest in the voting stock (or the equivalent) of such corporation of
other business. The transferees described in (a) through (e) above will be
required to agree in writing to be bound by the terms hereof as a condition of
any such transfer.

     Anything contained herein to the contrary notwithstanding, any person to
whom shares of Common Stock or Beneficially Owned Shares are transferred from
the undersigned shall be bound by the terms of this Agreement.


                                       27


<PAGE>


     In addition, the undersigned hereby waives, from the date hereof until the
expiration of the 90-day period following the date of the Company's final
Prospectus, any and all rights, if any, to request or demand registration
pursuant to the Securities Act of any shares of Common Stock that are registered
in the name of the undersigned or that are Beneficially Owned Shares. In order
to enable the aforesaid covenants to be enforced, the undersigned hereby
consents to the placing of legends and/or stop-transfer orders with the transfer
agent of the Common Stock with respect to any shares of Common Stock or
Beneficially Owned Shares.

                                     [Signatory]


                                     By:___________________________________
                                         Name:
                                         Title




                                       28



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