IMMUNOGEN INC
S-3, 1998-03-20
PHARMACEUTICAL PREPARATIONS
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<PAGE>   1
         AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MARCH 20, 1998

                                REGISTRATION NO.

                       SECURITIES AND EXCHANGE COMMISSION

                                    FORM S-3
                             REGISTRATION STATEMENT
                                      UNDER

                           THE SECURITIES ACT OF 1933

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

                                 IMMUNOGEN, INC.

             (Exact name of registrant as specified in its charter)

             MASSACHUSETTS                                 04-2726691
    (State or other jurisdiction of                     (I.R.S. Employer
     incorporation or organization)                    Identification No.)

       333 PROVIDENCE HIGHWAY, NORWOOD, MASSACHUSETTS 02062 (781) 769-4242
      (Address, including zip code, and telephone, including area code, of
                   registrant's principal executive offices)

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

                                 MITCHEL SAYARE
                            CHAIRMAN OF THE BOARD AND
                             CHIEF EXECUTIVE OFFICER
                                 IMMUNOGEN, INC.
                             333 PROVIDENCE HIGHWAY
                                NORWOOD, MA 02062
                                 (781) 769-4242
 (Name, address, including zip code, and telephone number, including area code, 
                             of agent for service)

                                    COPY TO:

                          JONATHAN L. KRAVETZ, ESQUIRE
               MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.
                              ONE FINANCIAL CENTER
                                BOSTON, MA 02111
                                 (617) 542-6000

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


<PAGE>   2


    Approximate date of commencement of proposed sale to public: As soon as
practicable after the effective date of this Registration Statement.

    If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box.

    If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, as amended (the "Securities Act"), other than securities offered only in
connection with dividend or interest reinvestment plans, check the following
box. X

    If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earliest
effective registration statement for the same offering.

    If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering.

    If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box.

                         CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>

====================================================================================================================================
                                                                            PROPOSED              PROPOSED
 TITLE OF EACH                                                              MAXIMUM               MAXIMUM              AMOUNT OF
 CLASS OF SECURITIES                                AMOUNT TO BE         OFFERING PRICE          AGGREGATE            REGISTRATION
 TO BE REGISTERED                                    REGISTERED            PER SHARE           OFFERING PRICE             FEE
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                                   <C>                  <C>                    <C>                   <C>    
 Common Stock, par value $.01 per share               475,425              $1.72 (1)              $817,731              $240.35
====================================================================================================================================
</TABLE>

(1) Estimated solely for the purpose of calculating the registration fee
    pursuant to Rule 457(c) of the Securities Act, based upon the average of the
    high and low sale prices of the Common Stock par value $.01 per share, of
    ImmunoGen, Inc. (the "Common Stock"), as reported on the Nasdaq National
    Market on March 18, 1998.

    THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNITL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933, AS AMENDED, OR UNTIL THE REGISTRATION STATEMENT
SHALL BECOME EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID
SECTION 8(a), MAY DETERMINE.


<PAGE>   3


                                   PROSPECTUS

                                 IMMUNOGEN, INC.
                         475,425 SHARES OF COMMON STOCK
                          (PAR VALUE OF $.01 PER SHARE)

The 475,425 shares of Common Stock of ImmunoGen, Inc., a Massachusetts
corporation ("ImmunoGen" or the "Company"), offered hereby are being sold by the
selling stockholders identified herein (the "Selling Stockholders"). Such offers
and sales may be made on one or more exchanges, in the over-the-counter market,
or otherwise, at prices and on terms then prevailing, or at prices related to
the then-current market price, or in negotiated transactions, or by underwriters
pursuant to underwriting agreements in customary form, or in a combination of
any such methods of sale. The Selling Stockholders may also sell such shares in
accordance with Rule 144 under the Securities Act. The Selling Stockholders are
identified and certain information with respect to them is provided under the
caption "Selling Stockholders" herein, to which reference is made. The expenses
of the registration of the securities offered hereby, including fees of counsel
for the Company, will be paid by the Company. The following expenses will be
borne by the Selling Stockholders: underwriting discounts and selling
commissions, if any, and the fees of legal counsel, if any, for the Selling
Stockholders. The filing by the Company of this Prospectus in accordance with
the requirements of Form S-3 is not an admission that any person whose shares
are included herein is an "affiliate" of the Company.

The Selling Stockholders have advised the Company that they have not engaged any
person as an underwriter or selling agent for any of such shares, but may in the
future elect to do so, and they will be responsible for paying such a person or
persons customary compensation for so acting. The Selling Stockholders and any
broker executing sell orders on behalf of any Selling Stockholder may be deemed
to be "underwriters" within the meaning of the Securities Act, in which event
commissions received by any such broker may be deemed to be underwriting
commissions under the Securities Act. The Company will not receive any of the
proceeds from the sale of the securities offered hereby. The Common Stock is
listed on the Nasdaq Stock Market ("Nasdaq") under the symbol IMGN. On March 13,
1998, the closing sale price of the Common Stock, as reported by Nasdaq, was 
$1.6875 per share.

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

         THE COMMON STOCK OFFERED HEREBY INVOLVES A HIGH DEGREE OF RISK.
                SEE "RISK FACTORS" ON PAGE 4 OF THIS PROSPECTUS.

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

    THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
       AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS
         THE SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES
             COMMISSION PASSED UPON THE ACCURACY OR ADEQUACY OF THIS
                 PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY
                             IS A CRIMINAL OFFENSE.

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

    No person is authorized in connection with any offering made hereby to give
any information or to make any representations other than as contained in this
Prospectus, and, if given or made, such information or representations must not
be relied upon as having been authorized by the Company. This Prospectus is not
an offer to sell, or a solicitation of an offer to buy, by any person in any
jurisdiction in which it is unlawful for such person to make such an offer or
solicitation. Neither the delivery of this Prospectus nor any sales made
hereunder shall under any circumstances create any implication that the
information contained herein is correct as of any time subsequent to the date
hereof.

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

                     THE DATE OF THIS PROSPECTUS IS MARCH 20, 1998.
<PAGE>   4

                              AVAILABLE INFORMATION

The Company is subject to the reporting requirements of the Securities Exchange
Act of 1934, as amended (the "1934 Act"), and in accordance therewith files
reports and other information with the Securities and Exchange Commission (the
"Commission"). These reports, proxy statements and other information can be
inspected and copied at the public reference facilities maintained by the
Commission at Room 1024 of the Commission's office at 450 Fifth Street, N.W.,
Judiciary Plaza, Washington, D.C. 20549, and at its regional offices located at
7 World Trade Center, Suite 1300, New York, NY 10048 and Citicorp Center, 500
West Madison Street, Suite 1400, Chicago, IL 60661. Copies of such reports,
proxy statements and other information can be obtained from the Public Reference
Section of the Commission at 450 Fifth Street, N.W., Washington, D.C. 20549 at
prescribed rates. The Commission maintains a Web site that contains reports,
proxy and information statements and other information regarding registrants
that file electronically with the Commission. The address of the Commission's
Web site is http://www.sec.gov. Additional updating information with respect to
the securities covered herein may be provided in the future to purchasers by
means of appendices to this Prospectus.

The Company has filed with the Commission in Washington, D.C. a registration
statement (herein, together with all amendments and exhibits, referred to as the
"Registration Statement") under the Securities Act with respect to the
securities offered or to be offered hereby. This Prospectus does not contain all
of the information included in the Registration Statement, certain items of
which are omitted in accordance with the rules and regulations of the
Commission. For further information about the Company and the securities offered
hereby, reference is made to the Registration Statement and the exhibits
thereto.

The Company will provide, without charge to each person to whom this Prospectus
is delivered, on the written or oral request of such person, a copy of any
document incorporated herein by reference, excluding exhibits. Requests should
be made to ImmunoGen, Inc., 333 Providence Highway, Norwood, MA 02062, telephone
(781) 769-4242 and directed to the attention of the Treasurer.

                                      - 2 -


<PAGE>   5




                                TABLE OF CONTENTS

                                                                  PAGE
                                                                  ----
    RISK FACTORS                                                 -  4 -
    THE COMPANY                                                  - 10 -
    SELLING STOCKHOLDERS                                         - 10 -
    PLAN OF DISTRIBUTION                                         - 11 -
    LEGALITY OF COMMON STOCK                                     - 11 -
    EXPERTS                                                      - 11 -
    INCORPORATION OF CERTAIN INFORMATION BY REFERENCE            - 12 -
    PART II. INFORMATION NOT REQUIRED IN PROSPECTUS               II-1
    SIGNATURES                                                    II-4






                                           -3-


<PAGE>   6




                                  RISK FACTORS

An investment in the shares being offered by this Prospectus involves a high
degree of risk. The following factors, in addition to those discussed elsewhere
in the Prospectus or incorporated herein by reference, should be carefully
considered in evaluating the Company and its business prospects before
purchasing shares offered by this Prospectus. This Prospectus contains and
incorporates by reference forward-looking statements within the "safe harbor"
provisions of the Private Securities Litigation ReForm Act of 1995. Reference is
made in particular to the discussion set forth under "Management's Discussion
and Analysis of Financial Condition and Results of Operations" in the Company's
Annual Report on Form 10-K for the fiscal year ended June 30, 1997 (the "Form
10-K") and the Quarterly Reports on Form 10-Q for the quarters ended September
30, 1997 (as amended by Amendment No.1 on Form 10-Q/A dated November 24, 1997)
and December 31, 1997, and under "Business" in the Form 10-K, incorporated in
this Prospectus by reference. Such statements are based on current expectations
that involve a number of uncertainties including those set forth in the risk
factors below. Actual results could differ materially from those projected in
the forward looking statements.

NEED FOR ADDITIONAL FUNDS AND ACCESS TO CAPITAL FUNDING. The Company's cash
resources at March 1, 1998 were approximately $2.2 million. This amount includes
cash received by Apoptosis Technology, Inc. ("ATI"), the Company's 96.5%-owned
subsidiary, from its collaborator, BioChem Pharma, Inc. ("BioChem"); $225,000
received in October 1997 under a $750,000 grant from the Small Business
Innovation Research ("SBIR") Program of the National Cancer Institute to advance
development over a two-year period of the Company's lead product candidate,
huC242-DM1; and $1.0 million received in December 1997 from the sale of the
Company's Series E Convertible Preferred Stock to an institutional investor in a
$3.0 million transaction expected to be completed in two additional
installments: $500,000 to be received in March 1998 and the remaining $1.5
million to be received on or before July 31, 1998, subject to the terms of the
agreement. Based on the Company's current operations, the Company believes that
its present funds, together with additional issuances of Series E stock, should
be sufficient to fund the Company's operations through at least October, 1998.
In addition, the Company is actively engaged in discussions with third parties
regarding potential financing and/or strategic partnering arrangements involving
an equity investment or other funding in the Company by such third parties.
However, there can be no assurance that these discussions will result in a
completed transaction. Any such additional funding may result in significant
dilution to the Company's current stockholders. If the Company is unable to
obtain financing on acceptable terms in the future or obtain funds through
arrangements with collaborative partners, which may require the Company to
relinquish certain material rights to its technology, it could be forced to
further scale back or discontinue its operations.

HISTORY OF OPERATING LOSSES AND ACCUMULATED DEFICIT. The Company has been
unprofitable since inception and expects to incur additional net losses over the
next several years, assuming it is able to raise sufficient working capital to
continue operations. The amount of net losses and the time required by the
Company to reach sustained profitability are highly uncertain. To achieve
profitability, the Company must, among other things, successfully complete
development of its products, obtain regulatory approvals and establish
manufacturing and marketing capabilities. There can be no assurance that the
Company will be able to achieve profitability at all or on a sustained basis.

UNCERTAINTIES ASSOCIATED WITH CLINICAL TRIALS. The Company has conducted and
plans to continue to undertake extensive and costly clinical testing to assess
the safety and efficacy of its potential products. The rate of completion of the
Company's clinical trials is dependent upon, among other factors, the rate of
patient enrollment. Patient enrollment is a function of many factors, including
the nature of the Company's clinical trial protocols, existence of competing
protocols, size of the patient population, proximity of patients to clinical
sites and eligibility criteria for the study. Delays in patient enrollment will
result in increased costs and delays, which could have a material adverse effect
on the Company. The Company cannot assure that patients enrolled in the
Company's clinical trials will continue in the trials or will respond to the
Company's product candidates. Setbacks are to be expected in conducting human
clinical trials. Failure to comply with the United States Food and Drug
Administration (the"FDA") regulations applicable to such testing can result in
delay, suspension or cancellation of such testing, and/or refusal by the FDA to
accept the results of such testing. In addition, the FDA may suspend clinical
trials at any time if it concludes that the subjects or patients participating
in such trials are being exposed to unacceptable health risks. Alternatively,
the FDA may require additional trials if it finds that the results of the
ongoing trials are inconclusive. Further, there can be no assurance that human
clinical testing will show any current or future product candidate to be safe
and effective or that data derived therefrom will be suitable for submission to
the FDA. See "Business--Regulatory Issues," in the Company's Form 10-K.

                                       -4-
<PAGE>   7

EARLY STAGE OF INITIAL PRODUCT DEVELOPMENT. The Company has not begun to market
or generate revenues from the sale of products. In March 1997, the Company
discontinued development of its lead product candidate, Oncolysin B, based on
preliminary data from the Phase III clinical trial. The Company intends to focus
its resources on potential products under development for the treatment of
colorectal cancer and small-cell lung cancer, now in research and preclinical
development, and on drug development based on proprietary apoptosis screens. The
Company's products will require significant additional development, laboratory
and clinical testing and investment prior to commercialization. There can be no
assurance that such products will be successfully developed, prove to be safe
and efficacious in clinical trials, meet applicable regulatory standards, obtain
required regulatory approvals, be capable of being produced in commercial
quantities at reasonable costs or be successfully marketed.

MANUFACTURING RISKS. The Company has not yet commercially introduced any
products. To be successful, the Company's products must be manufactured in
commercial quantities, in compliance with regulatory requirements and at
acceptable costs. Although the Company has produced its products in the
laboratory and scaled its production process to pilot levels, production in
commercial quantities will create substantial technical as well as financial
challenges for the Company. The Company's current facilities are not yet
approved by the FDA for commercial production of its proposed products, and
there can be no assurance that such approval will be obtained. In order to
manufacture its products in commercial quantities, the Company will have to
enhance its existing manufacturing facilities, which will require additional
funds, or rely on the manufacturing expertise of a strategic partner or contract
manufacturer. The Company has no experience in large-scale manufacturing, and no
assurance can be given that the Company will be able to make the transition to
commercial production successfully.

LACK OF MARKETING AND DISTRIBUTION EXPERIENCE. Although the Company may market
certain of its products through a direct sales force if and when regulatory
approval is obtained, it currently has no marketing or sales staff. To the
extent that the Company determines not to, or is unable to, arrange third-party
distribution for its products, significant additional expenditures, management
resources and time will be required to develop a sales force. There can be no
assurance that the Company will be able to establish such a sales force or be
successful in gaining market acceptance for its products.

RISK OF TECHNOLOGICAL CHANGE AND COMPETITION. The biotechnology industry is
subject to rapid and significant technological change. Competitors of the
Company engaged in all areas of biotechnology in the United States and abroad
are numerous and include major pharmaceutical and chemical companies,
specialized biotechnology firms, universities and other research institutions.
There can be no assurance that the Company's competitors will not succeed in
developing technologies and products that are more effective than any which have
been or are being developed by the Company or which would render the Company's
technology and products obsolete and noncompetitive. The Company's competitors
may also succeed in obtaining patent protection in other intellectual property
rights that would block the Company's ability to develop its proposed products.
Many of these competitors have substantially greater financial and technical
resources and production and marketing capabilities than the Company. In
addition, many of the Company's competitors have significantly greater
experience than the Company in preclinical testing and human clinical trials of
new or improved pharmaceutical products and in obtaining the FDA and other
regulatory approvals of products for use in health care. The Company has limited
experience in conducting and managing preclinical and clinical testing necessary
to obtain government approvals. Accordingly, the Company's competitors may
succeed in obtaining FDA approval for products more rapidly than the Company. If
the Company commences significant commercial sales of its products, it will also
be competing with respect to manufacturing efficiency and marketing
capabilities, areas in which it currently has no experience.

DEPENDENCE ON PATENTS AND PROPRIETARY RIGHTS. The patent situation in the field
of biotechnology generally is highly uncertain and involves complex legal,
scientific and factual questions. To date, no consistent policy has emerged
regarding the breadth of claims allowed in biotechnology patents. Accordingly,
there can be no assurance that patent applications relating to the Company's
products or technology will result in patents being issued or that, if issued,
the patents will afford protection against competitors with similar technology.

There has been significant litigation in the biotechnology industry regarding
patent and other intellectual property rights. This litigation is likely to
continue in the future. If the Company becomes involved in such litigation, it
could consume a substantial portion of the Company's resources. Also, patents
and applications owned or licensed by the Company may become the subject of
interference proceedings in the United States Patent and Trademark Office to
determine priority of invention, which could result in substantial cost to the
Company, as well as a possible adverse decision as to priority of invention of
the patent or patent application involved. An adverse decision in an
interference proceeding may result in the Company's loss of rights under a
patent or patent application subject to such a proceeding. In addition,
companies may obtain patents claiming products or processes that are necessary
for or useful to the development of the Company's products and bring legal
actions against the Company claiming infringement and

                                       -5-
<PAGE>   8

may seek to recover damages and to enjoin the Company from manufacturing and
marketing the affected product or process. If any such actions are successful,
in addition to any potential liability for damages, the Company may be required
to obtain licenses from others to continue to develop, manufacture or market its
products. There can be no assurance that the Company will prevail in any such
action or that it will be able to obtain such licenses on commercially
reasonable terms.

The Company and its subsidiary, ATI, seek patent protection for their
proprietary technology and products both in the United States and abroad. The
Company has received two United States patents and one European patent claiming
the use of maytansinoids in conjugated form as an invention, two United States
patents claiming use of DC1 and its analogs in immunoconjugates and one United
States patent claiming methods and use of its resurfacing technology. ATI has
received two United States patents. One patent claims the GD (BH3) domain as a
molecular target for the development of drugs which regulate apoptosis and one
patent claims specific antibodies to the apoptosis-related protein, Bcl-Y (also
referred to as Bak).

In addition, four patents have been issued to Dana-Farber Cancer Institute
("Dana-Farber") in the United States covering immunoconjugate technology
exclusively licensed by ImmunoGen from Dana-Farber. Two of these patents claim a
monoclonal antibody specific to small-cell lung carcinoma cells as an invention
and two of these patents claim the use of blocked ricin in immunoconjugates. A
patent assigned to Dana-Farber and exclusively licensed to ATI, claiming
composition of TIA 1 binding proteins, was allowed. Nine additional Dana-Farber
patents had been exclusively licensed to ImmunoGen, or ATI, and, at the
Company's option, have reverted back to Dana-Farber.

In order to practice its antibody humanization technology using either
Complementarity Determining Region ("CDR") grafting or resurfacing, the Company
may need to obtain one or more licenses under patents issued to third parties.
The Company understands that such licenses may be available on what it believes
to be commercially acceptable terms. However, there can be no assurance that any
such licenses will in fact be, or continue to be, available on commercially
acceptable terms, if at all.

The Company also relies upon unpatented proprietary technology. No assurance can
be given that others will not duplicate or independently develop substantially
equivalent technology, or otherwise gain access to the Company's proprietary
technology or disclose such technology, or that the Company can meaningfully
protect its rights in such unpatented proprietary technology.

DEPENDENCE ON OTHERS. The Company plans to conduct certain aspects of its future
operations with third-party collaborators who may fund collaborative work with
the Company. While the Company believes its potential collaborators will have an
economic motivation to succeed in performing their obligations under such
arrangements, the amount and timing of funds and other resources to be devoted
under such arrangements will be controlled by such other parties and would be
subject to financial or other difficulties that may befall such other parties.
Thus, no assurance can be given that the Company will generate any revenues from
such arrangements. In addition, although the Company is currently exploring
entry into such arrangements, no such arrangements have been concluded nor is
there any assurance that any such arrangements will ever come into effect.

DEPENDENCE ON KEY PERSONNEL. The Company's success is dependent on certain key
management and scientific personnel. Competition for qualified employees among
biotechnology companies is intense, and the loss of key personnel, or the
inability to attract and retain the additional, highly skilled employees
required for the expansion of the Company's activities, could adversely affect
its business, financial condition and results of operation.

GOVERNMENT REGULATION. The production and marketing of the Company's products
and its ongoing research and development activities are subject to regulation by
numerous governmental authorities in the United States and other countries. The
rigorous preclinical and clinical testing requirements and regulatory approval
processes typically take a number of years and require the expenditure of
substantial resources. Delays in obtaining regulatory approvals would adversely
affect the marketing of products developed by the Company and the Company's
ability to receive product revenues or royalties. In light of the limited
regulatory history of monoclonal antibody-based therapeutics, there can be no
assurance that regulatory approvals for the Company's products will be obtained
without lengthy delays, if at all. Moreover, the Company is, or may become,
subject to various federal, state and local laws, regulations and
recommendations relating to safe working conditions, laboratory and
manufacturing practices, the experimental use of animals and the use and
disposal of hazardous substances, including radioactive compounds and infectious
disease agents, used in connection with the Company's research work. In
addition, the Company cannot predict the extent to which existing or proposed
governmental regulations might have an adverse effect on the production and
marketing of the Company's products.

                                       -6-
<PAGE>   9

DEPENDENCE ON THIRD-PARTY REIMBURSEMENT. In both domestic and foreign markets,
sales of the Company's proposed products will depend in part on the availability
of reimbursement from third-party payors such as government health
administration authorities, private health insurers and other organizations.
Third-party payors are increasingly challenging the price and cost-effectiveness
of medical products and services. Significant uncertainty exists as to the
reimbursement status of newly approved health care products. There can be no
assurance that the Company's proposed products will be considered cost effective
or that adequate third-party reimbursement will be available to enable ImmunoGen
to maintain price levels sufficient to realize an appropriate return on its
investments in product development. Legislation and regulations affecting the
pricing of pharmaceuticals may change before any of the Company's proposed
products are approved for marketing. Adoption of such legislation and
regulations could further limit reimbursement for medical products and services.

HAZARDOUS MATERIALS; ENVIRONMENTAL MATTERS. The Company's research and
development activities involve the controlled use of hazardous materials,
chemicals, biological materials and radioactive compounds. The Company is
subject to federal, state and local laws and regulations governing the use,
manufacture, storage, handling and disposal of such materials and certain waste
products. Although the Company believes that its safety procedures for handling
and disposing of such materials comply with the standards prescribed by such
laws and regulations, the risk of accidental contamination or injury from these
materials cannot be completely eliminated. In the event of such an accident, the
Company could be held liable for any resulting damages, and any such liability
could exceed the Company's resources. The Company may be required to incur
significant costs to comply with environmental laws and regulations in the
future. Current or future environmental laws or regulation may have a material
adverse effect on the Company's business, financial condition and results of
operation.

PRODUCT LIABILITY EXPOSURE. The use of the Company's product candidates during
testing or after approval entails an inherent risk of adverse effects which
could expose the Company to product liability claims. There can be no assurance
that the Company would have sufficient resources to satisfy any liability
resulting from these claims. The Company currently has limited product liability
insurance for products which were in clinical testing. There can be no assurance
that such coverage will be adequate in scope to protect the Company in the event
of a successful product liability claim. Further, there can be no assurance that
the Company will be able to maintain such insurance or obtain general product
liability insurance on reasonable terms and at an acceptable cost once the
Company begins commercial production of its proposed products or that such
insurance will be in sufficient amounts to provide the Company with adequate
coverage against potential liabilities.

VOLATILITY OF STOCK PRICE. The market prices for securities of biotechnology
companies have been volatile. The market price for the Common Stock has
fluctuated significantly since public trading commenced in 1989, and it is
likely that the market price will continue to fluctuate in the future.
Announcements of technological innovations or new commercial products by the
Company or its competitors, developments concerning proprietary rights,
including patents and litigation matters, publicity regarding actual or
potential medical results relating to products under development by the Company
or its competitors, regulatory developments in both the United States and
foreign countries, public concern as to the safety of biotechnology products and
economic and other external factors, including the outbreak or material
escalation of hostilities or other calamity or crisis, as well as
period-to-period fluctuations in financial results, may have a significant
impact on the Company's business and on the market price of the Common Stock.
Sales of substantial amounts of the Common Stock in the public market may also
have an adverse impact on the market price of the Common Stock.

ABSENCE OF DIVIDENDS. The Company has not paid any cash dividends on its capital
stock since inception. Furthermore, the Company does not anticipate paying cash
dividends in the foreseeable future.

SHARES ELIGIBLE FOR FUTURE SALE. Sales of substantial amounts of Common Stock in
the public market could have an adverse effect on the price of the Common Stock.
In addition to the shares offered hereby, approximately 24,884,325 shares of
Common Stock were freely tradable on the open market as of March 1, 1998.
Further, approximately 874,270 shares are eligible for sale pursuant to Rule 144
of the Securities Act. Also, as of March 1, 1998, options to purchase 2,605,355
shares of Common Stock were outstanding pursuant to the Company's stock option
plans and 1,113,648 of such shares were vested and could be exercised at any
time prior to their respective expiration dates. As of March 1, 1998, 26,738
shares of Common Stock were issuable upon the exercise of warrants issued in
connection with a capital lease financing in March 1994, 2,102,299 shares of
Common Stock were issuable upon the exercise of warrants issued to date in
connection with the Company's March 1996 Financing (see below) and 2,597,117
shares of Common Stock were issuable upon the exercise of warrants issued to
date in connection with the Company's October 1996 Private Placement (see
below). Also as of March 1, 1998, warrants to purchase shares of Common Stock
equal to $3,538,000 invested to that

                                       -7-
<PAGE>   10
date in the Company's subsidiary, ATI, by BioChem have been issued, and 941,176
shares of Common Stock were issuable upon the exercise of warrants issued to
that date in connection with the Company's December 1997 Private Placement.

In March 1996, the Company issued $5.0 million principal amount convertible
debentures (the "Debentures") in a private placement (the "March 1996
Financing"). As part of the March 1996 Financing, the Company issued a $2.5
million principal amount debenture (the "First Debenture") on March 25, 1996. In
June 1996, the First Debenture, together with interest thereon, was converted
into shares of Common Stock, and warrants (the "First Warrant") to purchase
509,000 shares of Common Stock (the "First Warrant Shares") at an exercise price
of $4.00 per share were issued. The First Warrants expire in March 2001. In June
1996, a second $2.5 million convertible debenture (the "Second Debenture") was
issued and then converted into 2,500 shares of Series A Convertible Preferred
Stock ("Series A Stock") in October 1996. As of March 1, 1998, all 2,500 shares
of Series A Stock plus accrued interest thereon had been converted into
2,676,235 shares of the Company's Common Stock. In June 1996, the Company issued
additional warrants to purchase 500,000 shares of the Company's Common Stock
(the "Additional Warrants") in connection with the conversion of the First
Debenture into Common Stock. The Additional Warrants have an exercise price
equal to $6.00 per share and expire in March 2001. Additionally, warrants to
purchase 250,000 shares of the Company's Common Stock were issued as finder's
fees in connection with the issuance of the Debentures. Upon conversion of the
Series A Stock, the holder received warrants (the "Second Warrants") to purchase
a number of shares of Common Stock equal to 50% of the number of shares issuable
upon conversion of the Series A Stock. The Second Warrants are exercisable at
$4.00 per share and expire five years after the date of issuance. As of March 1,
1998, warrants to purchase 1,338,117 shares of the Company's Common Stock were
issued on conversion of the Series A Stock. There can be no assurance, however,
that any or all of the warrants will be exercised, or that the Company will
receive any proceeds from such exercise. The Company has registered for resale
3,877,000 shares of Common Stock in connection with the Series A Stock, the
First and Second Warrants, the Additional Warrant and the Warrants issued as a
finder's fee.

In October 1996, the Company sold $3.0 million of Series B Convertible Preferred
Stock ("Series B Stock") in connection with the October 1996 Private Placement.
As of March 1, 1998, all 3,000 shares of the Series B Stock plus accrued
dividends thereon had been converted into 1,384,823 shares of the Company's
Common Stock. In connection with the issuance of the Series B Stock, warrants to
purchase 500,000 shares of the Company's Common Stock were also issued. Of
these, 250,000 warrants (the "October 1996 Warrants") are exercisable at $5.49
per share and expire in October 2001. The remaining 250,000 warrants are
exercisable at $3.68 per share and expire in January 2002. There can be no
assurance that any or all of the warrants will be exercised, or that the Company
will receive any proceeds from such exercise. The Company has registered for
resale 1,916,666 shares of Common Stock in connection with the Series B Stock
and October 1996 Warrants.

In January 1997, the Company sold $3.0 million of Series C Convertible Stock
("Series C Stock") in connection with the October 1996 Private Placement. As of
March 1, 1998, all 3,000 shares of the Series C Stock plus accrued dividends
thereon had been converted into 2,719,738 shares of the Company's Common Stock.
In connection with the Series C Stock, 1,147,754 warrants to purchase Common
Stock were issued to the investor ("the April 1997 Warrants"). These warrants
are exercisable at $2.31 per share and expire in April, 2002. There can be no
assurance that any or all of the warrants will be exercised, or that the Company
will receive any proceeds from such exercise. The Company has registered for
resale 6,000,000 shares of Common Stock in connection with the Series C Stock
and April 1997 Warrants.

In June 1997, the Company sold $1.0 million of Series D Convertible Preferred
Stock ("Series D Stock") in connection with the October 1996 Private Placement.
As of March 1, 1998, all 1,000 shares of the Series D Stock plus accumulated
dividends thereon had been converted into 1,001,387 shares of the Company's
Common Stock. In connection with the Series D Stock 454,545 warrants to purchase
Common Stock were issued to the investor ("the September 1997 Warrants"). These
warrants are exercisable at $1.94 per share and expire in September 2002. There
can be no assurance that any or all of the warrants will be exercised, or that
the Company will receive any proceeds from such exercise. The Company has
registered for resale 2,757,862 shares of Common Stock in connection with the
Series D Stock and September 1997 Warrants.

In July 1997, ATI entered into a collaboration agreement with BioChem. The
agreement grants BioChem an exclusive, worldwide license to ATI's proprietary
screens based on two families of proteins involved in apoptosis, for use in
identifying leads for anti-cancer drug development. The agreement also covers
the development of new screens in two areas. Under the agreement, BioChem will
invest a total of $11.125 million in non-voting convertible preferred stock of
ATI in a series of private placements over a three-year period to be used
exclusively to fund research conducted under the collaboration during a
three-year research term. As of March 1, 1998, $3.538 million had been paid
under the agreement. The balance of $7.587 million will be paid in equal
quarterly installments of $843,000. The preferred stock is convertible into ATI
common stock at any time after three years from the date of first issuance of

                                       -8-
<PAGE>   11

such stock, at a conversion price equal to the then current market price of the
ATI common stock, but in any event at a price that will result in BioChem
acquiring at least 15% of the then outstanding ATI common stock. As part of this
agreement, BioChem also receives warrants to purchase shares of ImmunoGen Common
Stock equal to the amount invested in ATI during the three-year research term.
These warrants will be exercisable for a number of shares of ImmunoGen Common
Stock determined by dividing the amount of BioChem's investment in ATI by the
market price of the ImmunoGen Common Stock on the exercise date, subject to
certain limitations. The exercise price is payable either in cash or shares of
ATI preferred stock, at BioChem's option. The warrants are expected to be
exercised only in the event that the shares of ATI common stock do not become
publicly traded. In the event that ATI common stock does not become publicly
traded, the Company expects that BioChem will use its shares of ATI preferred
stock, in lieu of cash, to exercise the warrants.

In December 1997, the Company entered into an agreement to sell $3.0 million of
its Series E Convertible Preferred Stock ("Series E Stock") to an institutional
investor. The investment is expected to be completed in three installments. As
of March 1, 1998, $1.0 million had been received; the remainder is expected to
be received as follows, $500,000 to be received in March 1998 and the remaining
$1.5 million to be received on or before July 31, 1998, subject to the terms of
the agreement. The Series E Stock will be convertible into Common Stock at the
end of a two-year holding period at $1.0625 per share. Under the terms of the
agreement, the investor is entitled to receive warrants equal to 100% of the
number of shares of Common Stock issuable on conversion of the Series E Stock.
As of March 1, 1998, warrants to purchase 941,176 shares of Common Stock had
been issued. These warrants have an exercise price of $2.125 per share and
expire in 2004. There can be no assurance that any or all of the warrants will
be exercised, or that the Company will receive any proceeds from such exercise.
The holders of the Series E Stock have certain rights to register Common Stock
issued in connection with the sale of the Series E Stock under the Securities
Act for sale to the public, subject to certain conditions and limitations. These
registration rights can be exercised for a period of two years following the
first issuance of registrable securities. Only one such request for registration
of Common Stock can be made with respect to each sale of Series E Stock. In
addition, on completion of the sale of all $3.0 million of Series E Stock, the
Company will issue 75,000 shares of Common Stock to a third party as a finder's
fee.

The holders of approximately 792,769 shares of Common Stock (the "Registrable
Securities") are entitled to certain rights to register such shares under the
Securities Act for sale to the public, pursuant to a Registration Rights
Agreement by and among the Company and the holders of Registrable Securities, as
amended (the "Registration Rights Agreement"). The holders of Registrable
Securities include Aeneas Venture Corporation. Such holders have the right to
require the Company, on not more than two occasions, whether or not the Company
proposes to register any of its Common Stock for sale, to register all or part
of their shares for sale to the public under the Securities Act, subject to
certain conditions and limitations. In addition, holders of Registrable
Securities may require the Company to register all or part of their shares on
Form S-3 (or a successor short Form or registration) if the Company then
qualifies for use of such form, subject to certain conditions and limitations.
The Registration Rights Agreement was amended on October 9, 1991 to limit the
circumstances pursuant to which the registration rights granted thereunder may
be transferred to third parties and to amend certain procedural requirements.

In addition, pursuant to registration rights agreements between the Company and
holders of the Company's Series A Convertible Preferred Stock, Series B
Convertible Preferred Stock, Series C Convertible Preferred Stock and Series D
Convertible Preferred Stock (the "Preferred Stock") and/or related warrants (the
"Warrants"), such holders are entitled to rights to require the Company to
register for resale to the public under the Securities Act all shares of Common
Stock issued or issuable to such holders on conversion of the Preferred Stock
and /or exercise of the Warrants. The number of shares and warrants issued to
the preferred shareholder is determined according to a predetermined formula. As
of March 1, 1998, approximately 4,699,416 shares of Common Stock are
beneficially held by such holders.

As part of the agreement entered into in July 1997 between the Company, its
subsidiary, ATI, and BioChem (the "holder"), the holder receives warrants to
purchase shares of the Company's Common Stock equal to the amount invested in
ATI over a three-year period. These warrants become exercisable at the end of
the three-year period at the then current market price of the Common Stock.
Pursuant to a registration rights agreement between the Company and the holder,
at the end of the three-year period the holder is entitled to certain rights to
require the Company to register for sale to the public under the Securities Act
all registrable securities. As of March 1, 1998, amounts to purchase Common
Stock equal to the $3.538 million invested as of that date have been issued to
the holder.

DILUTION. Dilution is likely to occur upon conversion of the Company's
outstanding preferred stock, and also upon the exercise of outstanding stock
options and warrants. See "Shares Eligible for Future Sale."

                                       -9-
<PAGE>   12

                                   THE COMPANY

ImmunoGen develops pharmaceuticals, primarily for the treatment of cancer. In
March 1997, the Company discontinued development of its lead product candidate,
Oncolysin B, based on preliminary data from the Phase III clinical trial. The
Company intends to focus its resources on potential products under development
for the treatment of colorectal cancer and small-cell lung cancer, now in
research and preclinical development, and on drug development based on
proprietary apoptosis screens.

Through its majority-owned subsidiary, ATI, the Company is developing additional
technology platforms based on the regulation of programmed cell death, or
apoptosis. ATI is applying its understanding of how apoptotic pathways are
triggered in cells to identify product candidates for the treatment of cancer
and viral infections, two targets where inhibition of apoptosis is recognized as
an essential element of the disease. ATI has identified several key proteins
which play a role in the regulation of apoptosis in cancer cells and viruses
and, using these, has developed proprietary screens with which to identify leads
for drug development.

The Company was organized in 1981 as a Massachusetts corporation. The Company's
principal offices are located at 333 Providence Highway, Norwood, Massachusetts
02062, and its telephone number is (781) 769-4242.

                              SELLING STOCKHOLDERS

The shares of Common Stock offered hereby by the Selling Stockholders were
issued by the Company to the Selling Stockholders pursuant to a Stock Purchase
Agreement dated as of January 11, 1993 between the Company, Dr. Stuart
Schlossman, Dana Farber and ATI.

The number of shares registered on the Registration Statement of which this
Prospectus is a part and the number of shares offered hereby have been
determined by agreement between the Company and the Selling Stockholders.

The following table sets forth information with respect to the beneficial
ownership of the Company's Common Stock by the Selling Stockholders as of March
1, 1998, and as adjusted to reflect the sale of the Common Stock offered hereby
by the Selling Stockholders.

<TABLE>
<CAPTION>

                                      SHARES            NUMBER OF               SHARES
                                  OWNED PRIOR TO       SHARES BEING           OWNED AFTER
SELLING STOCKHOLDER                  OFFERING             OFFERED             OFFERING(1)
- -------------------               --------------       ------------           -----------
                                                                          NUMBER      PERCENT
                                                                          ------      -------
<S>                                   <C>                <C>              <C>             <C>
Stuart F. Schlossman, M.D.(3)         246,294            237,713          8,581         * (2)

Stuart Schlossman 1994
  Irrevocable Trust dated as 
  of 9/26/94 - 
  Robert L. Schlossman Share          118,856            118,856             0            --

Stuart Schlossman 1994
  Irrevocable Trust dated as
  of 9/26/94 - 
  Peter E. Schlossman Share           118,856            118,856             0            --
</TABLE>

*  Less than 1%

(1) Assumes the sale of all shares offered hereby to unaffiliated third parties.

(2) Based on 24,884,325 shares of Common Stock outstanding on March 1, 1998.

(3) Dr. Stuart Schlossman disclaims beneficial ownership of shares owned by the
    trusts.

                                      -10-
<PAGE>   13

                              PLAN OF DISTRIBUTION

The 475,425 shares of Common Stock of the Company offered hereby may be offered
and sold from time to time by the Selling Stockholders, or by pledgees, donees,
transferees or other successors in interest. Such offers and sales may be made
from time to time on one or more exchanges or in the over-the-counter market, or
otherwise, at prices and on terms then prevailing or at prices related to the
then-current market price, or in negotiated transactions. The methods by which
the shares may be sold may include, but not be limited to, the following: (a) a
block trade in which the broker or dealer so engaged will attempt to sell the
shares as agent but may position and resell a portion of the block as principal
to facilitate the transaction; (b) purchases by a broker or dealer as principal
and resale by such broker or dealer for its account; (c) an exchange
distribution in accordance with the rules of such exchange; (d) ordinary
brokerage transactions and transactions in which the broker solicits purchasers;
(e) privately negotiated transactions; (f) short sales; and (g) a combination of
any such methods of sale. In effecting sales, brokers or dealers engaged by the
Selling Stockholders may arrange for other brokers or dealers to participate.
Brokers or dealers may receive commissions or discounts from the Selling
Stockholders or from the purchasers in amounts to be negotiated immediately
prior to the sale. The Selling Stockholders may also sell such shares in
accordance with Rule 144 under the Securities Act.

From time to time the Selling Stockholders may engage in short sales, short
sales against the box, puts and calls and other transactions in securities of
the Company or derivatives thereof, and may sell and deliver the shares in
connection therewith. From time to time Selling Stockholders may pledge their
shares pursuant to the margin provisions of their respective customer agreements
with their respective brokers. Upon a default by a Selling Stockholder, the
broker may offer and sell the pledged shares of Common Stock from time to time.
The Company has agreed to use its best efforts to maintain the effectiveness of
the registration of the shares being offered hereunder until March 2000 or such
earlier date when all of the shares being offered hereunder have been sold or
may be sold without volume or other restrictions pursuant to Rule 144 or Rule
144A under the Securities Act, as determined by counsel to the Company pursuant
to a written opinion letter.

The Selling Stockholders and any brokers participating in such sales may be
deemed to be underwriters within the meaning of the Securities Act. There can be
no assurance that the Selling Stockholders will sell any or all of the shares of
Common Stock offered hereunder.

All proceeds from any such sales will be the property of the Selling
Stockholders who will bear the expense of underwriting discounts and selling
commissions, if any, and its own legal fees.

                            LEGALITY OF COMMON STOCK

The validity of the shares of Common Stock offered hereby is being passed upon
for the Company by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., Boston,
Massachusetts.

                                     EXPERTS

The financial statements incorporated in this Prospectus by reference to the
Company's Annual Report on Form 10-K for the fiscal year ended June 30, 1997
have been so incorporated in reliance on the report (which includes an
explanatory paragraph concerning uncertainties surrounding the Company's ability
to continue as a going concern) of Coopers & Lybrand L.L.P., independent
accountants, given on the authority of said firm as experts in auditing and
accounting.


                                      -11-
<PAGE>   14
                INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The following documents filed with the Commission are incorporated herein by
reference:

(a) The Company's Annual Report on Form 10-K for the fiscal year ended June 30,
    1997.

(b) The Company's Quarterly Report on Form 10-Q for the quarter ended September
30, 1997, as amended by Amendment No. 1 dated November 24, 1997.

(c) The Company's Quarterly Report on Form 10-Q for the quarter ended December
31, 1997.

(d) The Company's Current Report on Form 8-K for the August 1, 1997 event.

(e) The Company's Current Report on Form 8-K for the December 11, 1997 event.

(f) The description of the Company's capital stock contained in the Company's
registration statement on Form 8-A under the 1934 Act (File No. 0-17999),
including amendments or reports filed for the purpose of updating such
description.

All reports and other documents subsequently filed by the Company with the
Commission pursuant to Sections 13(a), 13(c), 14 and 15(d) of the 1934 Act,
prior to the filing of a post-effective amendment which indicates that all
securities covered by this Prospectus have been sold or which deregisters all
such securities then remaining unsold, shall be deemed to be incorporated by
reference herein and to be a part hereof from the date of the filing of such
reports and documents.

                                      -12-


<PAGE>   15



                 PART II. INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

The following expenses incurred in connection with the sale of the securities
being registered will be borne by the Registrant. Other than the SEC
registration fee, the amounts stated are estimates.

<TABLE>

           <S>                                                <C>
           SEC Registration Fee                               $    245.44

           Legal Fees and Expenses                               2,000.00

           Accounting Fees and Expenses                          1,000.00

           Miscellaneous                                         7,500.00
                                                              -----------
                     TOTAL                                    $ 10,745.44
                                                              ===========
</TABLE>


The Selling Stockholder will bear the expense of its own legal counsel and
miscellaneous fees and expenses, if any.

ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

Article 6(d) of the Registrant's Restated Articles of Organization provides as
follows:

"(d) The liability of the Directors of the Corporation shall be limited to the
fullest extent permitted by Section 13(b)(1 1/2) of the Massachusetts Business
Corporation Law."

Section 6.6 of the Registrant's By-Laws provides as follows:

"Section 6.6 Indemnification of Officers, Directors, and Members of the
Scientific Advisory Board. The corporation shall indemnify and hold harmless
each person, now or hereafter an officer or Director of the corporation, or a
member of the Scientific Advisory Board, from and against any and all claims and
liabilities to which he may be or become subject by reason of his being or
having been an officer, Director of member of the Scientific Advisory Board of
the corporation or by reason of his alleged acts or omissions as an officer,
Director or member of the Scientific Advisory Board of the corporation, and
shall indemnify and reimburse each such officer, Director and member of the
Scientific Advisory Board against and for any and all legal and other expenses
reasonably incurred by him in connection with any such claims and liabilities,
actual or threatened, whether or not at or prior to the time which so
indemnified, held harmless and reimbursed he has ceased to be an officer,
Director or member of the Scientific Advisory Board of the corporation, except
with respect to any matter as to which such officer, Director or member of the
Scientific Advisory Board of the corporation shall have been adjudicated in any
proceeding not to have acted in good faith in the reasonable belief that his
action was in the best interest of the corporation; provided, however, that
prior to such final adjudication the corporation may compromise and settle any
such claims and liabilities and pay such expenses, if such settlement or payment
or both appears, in the judgment of a majority of those members of the Board of
Directors who are not involved in such matters, to be for the best interest of
the corporation as evidenced by a resolution to that effect adopted after
receipt by the corporation of a written opinion of counsel for the corporation,
that, based on the facts available to such counsel, such officer, Director or
member of the Scientific Advisory Board of the corporation has not been guilty
of acting in a manner that would prohibit indemnification.

Such indemnification may include payment by the corporation of expenses incurred
in defending a civil or criminal action proceeding in advance of the final
disposition of such action or proceeding, upon receipt of an undertaking by the
person indemnified to repay such payment if he shall be adjudicated not to be
entitled to indemnification under this section.

The corporation shall similarly indemnify and hold harmless persons who serve at
its express written request as directors or officers of another organization in
which the corporation owns shares or of which it is a creditor.

                                      II-1
<PAGE>   16

The right of indemnification herein provided shall be in addition to and not
exclusive of any other rights to which any officer, Director or member of the
Scientific Advisory Board of the corporation, or any such persons who serve at
its request as aforesaid, may otherwise be lawfully entitled. As used in this
Section, the terms "officer," "Director," and "member of the Scientific Advisory
Board" include their respective heirs, executors, and administrators.

ITEM 16. EXHIBITS.

     EXHIBIT
     NUMBER                       DESCRIPTION
     ------                       -----------

      4.1   Article 4 of the Restated Articles of Organization of the Registrant
            (previously filed as Exhibit No. 3.1 to the Registrant's
            Registration Statement on Form S-1, File No. 33-38883, and
            incorporated herein by reference)

      4.2   Form of Common Stock Certificate (previously filed as Exhibit No.
            4.2 to the Registrant's Registration Statement on Form S-1, File No.
            33-31219, and incorporated herein by reference)

      5     Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., with
            respect to the legality of the securities being registered

     23.1   Consent of Coopers & Lybrand L.L.P.

     23.2   Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
            (included in Exhibit 5)

     24     Power of Attorney (filed in Part II of this Registration Statement)

     99.1   Stockholders' Agreement dated as of January 11, 1993, by and among
            the Registrant, Dana-Farber Cancer Institute, Inc., Dr. Stuart
            Schlossman and Apoptosis Technology, Inc.

ITEM 17. UNDERTAKINGS.

A. RULE 415 OFFERING

The undersigned Registrant hereby undertakes:

(1) To file, during any period in which offers or sales are being made, a
post-effective amendment to this registration statement:

(i) To include any prospectus required by Section 10(a)(3) of the Securities
Act;

(ii) To reflect in the prospectus any facts or events arising after the
effective date of the registration statement (or the most recent post-effective
amendment thereof) which, individually or in the aggregate, represent a
fundamental change in the information set forth in the registration statement.
Notwithstanding the foregoing, any increase or decrease in volume of securities
offered (if the total dollar value of securities offered would not exceed that
which was registered) and any deviation from the low or high end of the
estimated maximum offering range may be reflected in the Form of prospectus
filed with the Commission pursuant to Rule 424(b) of Regulation C under the
Securities Act if, in the aggregate, the changes in volume and price represent
no more than a 20% change in the maximum aggregate offering price set forth in
the "Calculation of Registration Fee" table in the effective registration
statement.

(iii) To include any material information with respect to the plan of
distribution not previously disclosed in the registration statement or any
material change to such information in the registration statement;

                                      II-2
<PAGE>   17

PROVIDED, HOWEVER, that paragraphs

(1)(i) and (1)(ii) do not apply if the registration statement is on Form S-3 or
Form S-8, and the information required to be included in a post-effective
amendment by those paragraphs is contained in periodic reports filed by the
Registrant pursuant to Section 13 or Section 15(d) of the 1934 Act that are
incorporated by reference in the registration statement.

(2) That, for the purpose of determining any liability under the Securities Act,
each such post-effective amendment shall be deemed to be a new registration
statement relating to the securities offered therein, and the offering of such
securities at that time shall be deemed to be the initial bona fide offering
thereof.

(3) To remove from registration by means of a post-effective amendment any of
the securities being registered which remain unsold at the termination of the
offering.

B.  FILINGS INCORPORATING SUBSEQUENT EXCHANGE ACT DOCUMENTS BY REFERENCE

The undersigned Registrant hereby undertakes that, for purposes of determining
any liability under the Securities Act, each filing of the Registrant's annual
report pursuant to section 13(a) or section 15(d) of the 1934 Act (and, where
applicable, each filing of an employee benefit plan's annual report pursuant to
section 15(d) of the 1934 Act) that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

C.  REQUEST FOR ACCELERATION OF EFFECTIVE DATE OR FILING OF REGISTRATION
    STATEMENT ON FORM S-8

Insofar as indemnification for liabilities arising under the Securities Act may
be permitted to directors, officers and controlling persons of the Registrant
pursuant to the foregoing provisions, or otherwise, the Registrant has been
advised that in the opinion of the Commission such indemnification is against
public policy as expressed in the Securities Act and is, therefore,
unenforceable. In the event that a claim for indemnification against such
liabilities (other than the payment by the Registrant of expenses incurred or
paid by a director, officer or controlling person of the registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the Registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Securities Act and will be governed by the final
adjudication of such issue.

                                      II-3


<PAGE>   18



                                   SIGNATURES

Pursuant to the requirements of the Securities Act, as amended, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in Norwood, Massachusetts on March 19, 1998.

                                         IMMUNOGEN, INC.

                                         By: /s/ Mitchel Sayare
                                             ---------------------------------
                                             Mitchel Sayare, Chairman of the
                                             Board and Chief Executive Officer


                                POWER OF ATTORNEY

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

KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears below
constitutes and appoints Mitchel Sayare and Kathleen A. Carroll, or any of them,
his attorney-in-fact, each with the power of substitution, for him in any and
all capacities, to sign any and all amendments (including post-effective
amendments) to this registration statement, (or any other registration statement
for the same offering that is to be effective upon filing pursuant to Rule
462(b) under the Securities Act) and to file the same, with all exhibits thereto
and other documents in connection therewith, with the Securities and Exchange
Commission, hereby granting unto said attorneys-in-fact and agents, and each of
them, full power and authority to do and perform each and every act and thing
requisite or necessary to be done in and about the premises, as full to all
intents and purposes as he might or could do in person, hereby ratifying and
confirming all that said attorneys-in-fact and agents or any of them or their or
his substitutes may lawfully do or cause to be done by virtue hereof.

<TABLE>
<CAPTION>

          SIGNATURE                                      TITLE                            DATE

    <S>                              <C>                                             <C> 
    /s/ Mitchel Sayare               Chairman of the Board of Directors              March 19, 1998
    -----------------------          and Chief Executive Officer  
    Mitchel Sayare                   (principal executive officer)

    /s/ Kathleen A. Carroll          Vice President, Finance and Administration,     March 19, 1998
    -----------------------          Treasurer (principal financial officer
    Kathleen A. Carroll              and principal accounting officer)

    /s/ Walter A. Blattler           Executive Vice President,                       March 19, 1998
    -----------------------          Science and Technology and Director
    Walter A. Blattler 

    /s/ Michael R. Eisenson          Director                                        March 19, 1998
    -----------------------
    Michael R. Eisenson

    /s/ Stuart F. Feiner             Director                                        March 19, 1998
    -----------------------
    Stuart F. Feiner

    /s/ Donald E. O'Neill            Director                                        March 19, 1998
    -----------------------
    Donald E. O'Neill

    /s/ David W. Carter              Director                                        March 19, 1998
    -----------------------
    David W. Carter
</TABLE>

                                      II-4

<PAGE>   1
                                                                       EXHIBIT 5


               Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
                              One Financial Center
                           Boston, Massachusetts 02111

701 Pennsylvania Avenue, N.W.                            Telephone: 617/542-6000
Washington, D.C. 20004                                   Fax: 617/542-2241
Telephone: 202/434-7300
Fax: 202/434-7400




                                            March 20, 1998




ImmunoGen, Inc.
333 Providence Highway
Norwood, Massachusetts 02062

Ladies and Gentlemen:

       We have acted as counsel to ImmunoGen, Inc., a Massachusetts corporation
(the "Company"), in connection with the preparation and filing with the
Securities and Exchange Commission of a Registration Statement on Form S-3 (the
"Registration Statement"), pursuant to which the Company is registering under
the Securities Act of 1933, as amended, a total of 475,425 shares (the "Shares")
of its common stock, $.01 par value per share (the "Common Stock"), for resale
to the public. The Shares are to be sold by the selling stockholders identified
in the Registration Statement. This opinion is being rendered in connection with
the filing of the Registration Statement.

       In connection with this opinion, we have examined the Company's Restated
Articles of Organization and Restated By-Laws, both as currently in effect; such
other records of the corporate proceedings of the Company and certificates of
the Company's officers as we have deemed relevant; and the Registration
Statement and the exhibits thereto.

       In our examination, we have assumed the genuineness of all signatures,
the legal capacity of natural persons, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as certified or photostatic copies and the
authenticity of the originals of such copies.

     Based upon the foregoing, we are of the opinion that (i) the Shares have
been duly authorized and validly issued by the Company and (ii) the Shares, when
sold, will be duly authorized, validly issued, fully paid and non-assessable
shares of the Common Stock, free of preemptive rights.

       Our opinion is limited to the laws of the Commonwealth of Massachusetts,
and we express no opinion with respect to the laws of any other jurisdiction. No
opinion is expressed herein with respect to the qualification of the Shares
under the securities or blue sky laws of any state or any foreign jurisdiction.



<PAGE>   2

Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.



ImmunoGen, Inc.
March 20, 1998
Page 2


       We understand that you wish to file this opinion as an exhibit to the
Registration Statement, and we hereby consent thereto. We hereby further consent
to the reference to us under the caption "Legality of Common Stock" in the
prospectus included in the Registration Statement.

                                                 Very truly yours,



                                                 /s/ Mintz, Levin, Cohn, Ferris,
                                                     Glovsky and Popeo, P.C.
                                                 ------------------------------
                                                 MINTZ, LEVIN, COHN, FERRIS,
                                                  GLOVSKY AND POPEO, P.C.
 





<PAGE>   1
                                                                    EXHIBIT 23.1

                       CONSENT OF INDEPENDENT ACCOUNTANTS



      We consent to the incorporation by reference in this Registration
Statement of ImmunoGen, Inc. on Form S-3 to register 475,425 shares of common
stock of our report (which includes an explanatory paragraph concerning
uncertainties surrounding the Company's ability to continue as a going concern),
dated July 30, 1997, on our audits of the consolidated financial statements of
ImmunoGen, Inc. as of June 30, 1996 and 1997 and for each of the three years in
the period ended June 30, 1997, which report is included in the Company's 1997
Annual Report on Form 10-K.

      We also consent to the reference to our Firm in the Registration Statement
under the caption "Experts".



                                                /s/ Coopers & Lybrand L.L.P.
                                                ----------------------------
                                                COOPERS & LYBRAND L.L.P.


Boston, Massachusetts
March 19, 1998

<PAGE>   1
                                                                    EXHIBIT 99.1



                             STOCKHOLDERS' AGREEMENT



       This Agreement made as of this 11th day of January, 1993, by and among
ImmunoGen, Inc., a Massachusetts corporation with its principal place of
business at 148 Sidney Street, Cambridge, Massachusetts 02139 ("IMMUNOGEN"),
Dana-Farber Cancer Institute, Inc., a Massachusetts non-profit corporation with
its principal place of business at 44 Binney Street, Boston, Massachusetts 02115
("DFCI"), Dr. Stuart Schlossman, a Massachusetts resident residing at One Fox
Place, Newton Center, Massachusetts 02159 ("SCHLOSSMAN"; Schlossman and DFCI are
collectively referred to herein as the "STOCKHOLDERS" and individually as a
"STOCKHOLDER"), and Apoptosis Technology, Inc., a Massachusetts corporation with
its principal place of business at 148 Sidney Street, Cambridge, Massachusetts
02139("ATI").


       WHEREAS, ImmunoGen owns 7,000 shares of $.01 par value Class A Preferred
Stock (the "PREFERRED STOCK") of ATI and each of the Stockholders presently owns
1,000 shares of $.01 par value Common Stock (the "COMMON STOCK") of ATI; and


       WHEREAS, the purpose of this Agreement is to provide a mechanism to
enable the Stockholders to realize the value of their ownership interest in ATI
through certain liquidity arrangements and to place certain restrictions upon
all of the present and future shares of the Common Stock of ATI held or to be
held by the Stockholders.


       NOW, THEREFORE, in consideration of the foregoing and the mutual premises
contained herein and for other good and valuable consideration, the receipt and
adequacy of which is hereby acknowledged, the parties, intending to be legally
bound, agree as follows:



       SECTION 1.   DEFINITIONS.


       (A)    "Additional Stockholder Shares" -- Any issued and outstanding
shares of any class of equity securities of ATI, acquired by any of the
Stockholders after the date hereof.


       (B)    "Closing Date" -- The date on which the closing shall take place
on the sale and purchase of any Stockholder Shares (as defined below) pursuant
to this Agreement.


       (C)   "Stockholder Shares" -- All of the issued and outstanding shares of
Common Stock of ATI owned by any of the Stockholders as of the date hereof.





<PAGE>   2

       SECTION 2.   GENERAL RESTRICTIONS ON TRANSFER OR OTHER DISPOSITION OF THE
STOCKHOLDER SHARES.


       (A)   GENERAL RESTRICTIONS. No Stockholder shall sell, transfer, assign
or otherwise dispose of, pledge, hypothecate or otherwise encumber any of the
Stockholder Shares or Additional Stockholder Shares, whether voluntarily or by
operation of law, except as provided in this Agreement. Any purported sale,
transfer, assignment or other disposition or pledge, hypothecation or other
encumbrance in violation of this Agreement shall be void and ineffectual and
shall not operate to transfer any interest or title. All stock certificates for
any Stockholder Shares or Additional Stockholder Shares held by or to be
hereafter issued to any of the Stockholders shall contain a restrictive
endorsement subjecting the transfer, assignment or other disposition, or the
pledge, hypothecation or other encumbrance, of the Stockholder Shares and
Additional Stockholder Shares thereby represented to the terms and conditions of
this Agreement and each of the Stockholders shall surrender to ATI at the time
of execution of this Agreement all stock certificates for Stockholder Shares for
the purpose of placing such a legend on such certificates. In the event any
Stockholder shall fail to deliver to ImmunoGen any stock certificate
representing shares which such Stockholder is obligated to transfer to ImmunoGen
pursuant to the terms of this Agreement, then, upon deposit in a separate
account, which need not bear interest, of the purchase price for such shares as
determined in accordance with the terms of this Agreement, ATI shall be entitled
to treat such shares as transferred in accordance with the terms of this
Agreement, to treat such transferee as the owner of such shares for all purposes
and to deem cancelled the certificate or certificates issued in the name of such
Stockholder for such shares. The purchase price so deposited shall be paid to
such Stockholder from such account upon surrender by such Stockholder of the
certificate or certificates representing such shares. ATI shall not be required
to transfer any Stockholder Shares or Additional Stockholder Shares which shall
have been purportedly sold or transferred in violation of this Agreement or to
treat as the owner of such Stockholder Shares or Additional Stockholder Shares,
whether for purposes of voting such Stockholder Shares or Additional Stockholder
Shares, receiving any dividends or other distributions or for any other purpose,
any such purported transferee.


       (B)   PERMITTED TRANSFERS TO FAMILY. Nothing herein shall preclude the
gift of any of the Stockholder Shares or Additional Stockholder Shares held by
any Stockholder to his or her spouse, child, stepchild, grandchild, parent or
sibling, or legal dependent or to a trust of which the beneficiary or
beneficiaries of the corpus and the income shall be either such a






                                       2
<PAGE>   3

person or the Stockholder (a "Familial Transfer"). In any such case, applicable
references in this Agreement to a Stockholder shall be deemed to include each
permitted transferee of a Stockholder pursuant to a Familial Transfer, and in
each such case as a condition of such transfer, each recipient of such
Stockholder Shares or Additional Stockholder Shares shall be required to execute
a written instrument agreeing to be bound by the terms and provisions of this
Agreement in the same manner as the transferor of such Stockholder Shares or
Additional Stockholder Shares.



       SECTION 3.   RIGHT OF FIRST REFUSAL TO PURCHASE STOCKHOLDER SHARES AND
ADDITIONAL STOCKHOLDER SHARES.


       (A)   In the event that any Stockholder wishes to dispose of all or any
portion of his, her or its Stockholder Shares or Additional Stockholder Shares
(other than pursuant to a Familial Transfer) (the "Offered Shares"), such
Stockholder shall not offer any of such Stockholder Shares or Additional
Stockholder Shares to a third party unless and until such Stockholder shall have
complied with the provisions of this Subsection (A). Such Stockholder shall
first offer the Offered Shares for sale to ImmunoGen in a written notice to
ImmunoGen, whereupon such Stockholder and ImmunoGen shall enter into good faith
negotiations to determine the purchase price for the Offered Shares. If within
thirty (30) days of such notice, the selling Stockholder and ImmunoGen have not
reached agreement on the purchase price, then the selling Stockholder shall be
free to offer the Offered Shares to a third party, provided the Stockholder
complies with the provisions of Subsections (B), (C) and (D) below.


       (B)   In the event that any Stockholder wishes to dispose of all or any
portion of his or her Stockholder Shares or Additional Stockholder Shares (other
than pursuant to a Familial Transfer) and such Stockholder shall have complied
with Subsection (A) above, then such Stockholder may offer the Offered Shares to
a third party, subject to the provisions of this Subsection (B) and Subsections
(C) and (D) below. Upon the receipt by the selling Stockholder of a bona fide
written offer from a third party, the selling Stockholder shall simultaneously
notify ImmunoGen and ATI of the identities of the proposed purchaser or
purchasers, the number of Offered Shares and a copy of such written offer
setting forth the proposed price and terms of sale (the "Offer Notice"). Despite
any revocation of the third party written offer, the selling Stockholder shall,
at the option of ImmunoGen exercised in accordance with Subsection (C) below, be
obligated to sell the Offered Shares to ImmunoGen at the price and on the terms
of the third party written offer,





                                       3
<PAGE>   4

subject to compliance with the following provisions. For all purposes of this
Section 3, the "First Refusal Price" shall be the price (and upon the terms)
contained in the Offer Notice, provided, that in the event the consideration to
be paid pursuant to the Offer Notice includes anything other than United States
dollars, at ImmunoGen's option ImmunoGen may substitute United States dollars
equal to the fair value of such other consideration in lieu thereof. Such fair
value shall be conclusively determined in good faith by the Board of Directors
of ATI.


       (C)   ImmunoGen shall have a right of first refusal to purchase all, but
not less than all, of the Offered Shares at the First Refusal Price. ImmunoGen
shall exercise its right to purchase the Offered Shares by written notice to the
selling Stockholder within thirty (30) days following receipt of the Offer
Notice. The notice from ImmunoGen shall designate the place for closing in the
greater Boston area and the Closing Date, which shall be no later than sixty
(60) days following receipt of the Offer Notice.


       (D)   If ImmunoGen shall not have notified the selling Stockholder of its
intention to purchase all of the Offered Shares as provided in Subsection (C)
above, then the selling Stockholder shall be permitted during the thirty (30)
days after the first thirty (30) days after the receipt by ImmunoGen of the
Offer Notice to sell all, but not less than all, of the Offered Shares to the
same purchaser or purchasers, at the same price and on the same terms as set
forth in the Offer Notice, provided such purchaser or purchasers execute a
written instrument agreeing to be bound by the terms and provisions of this
Agreement in the same manner as the transferor of such Stockholder Shares;
provided, however, that the purchaser or purchasers shall not have the benefit
of, and shall not be entitled to exercise, the Put Option set forth in Section
4(A). If the selling Stockholder shall have received a notice from ImmunoGen,
but ImmunoGen shall have failed to tender the First Refusal Price on or before
the Closing Date specified in such notice, then the selling Stockholder shall be
permitted, for a period of thirty (30) days after such Closing Date, to sell all
of the Offered Shares to the same purchaser or purchasers, at the same price and
on the same terms as set forth in the Offer Notice, provided such purchaser or
purchasers execute a written instrument agreeing to be bound by the terms and
provisions of this Agreement in the same manner as the transferor of such
Stockholder Shares or Additional Stockholder Shares; provided, however, that the
purchaser or purchasers shall not have the benefit of, and shall not be entitled
to exercise, the Put Option set forth in Section 4(A).




                                       4
<PAGE>   5

       SECTION 4.   RIGHT TO SELL AND PURCHASE SHARES.


       (A)   PUT OPTION.  In the event that, prior to January 11, 1998, ATI
shall not have sold securities to the public in an offering registered under the
Securities Act of 1933, as amended (or successor legislation) in which the total
offering price is at least $5,000,000, then, at any time on or after January 11,
1998 but before January 11, 1999, upon the written request of a Stockholder to
ImmunoGen, said Stockholder shall have the right to sell, and ImmunoGen shall be
obligated to purchase, all but not less than all of the Stockholder Shares owned
by said Stockholder to ImmunoGen (the "Put Option"). The purchase price per
Stockholder Share shall be determined as set forth in Subsection (C) below and
the Closing Date shall be determined in accordance with Subsection (D) below.


       (B)   CALL OPTION.  In the event that, prior to January 11, 1998, ATI
shall not have sold securities to the public in an offering registered under the
Securities Act of 1933, as amended (or successor legislation) in which the total
offering price is at least $5,000,000, then, at any time on or after January 11,
1998 but before January 11, 1999, ImmunoGen shall have the right to purchase,
and each of the Stockholders shall be obligated to sell, all but not less than
all of the Stockholder Shares owned by each Stockholder upon written notice by
ImmunoGen to one or more of such Stockholders (the "Call Option"). The purchase
price per Stockholder Share shall be determined as set forth in Subsection (C)
below and the Closing Date shall be determined in accordance with Subsection (D)
below.


       (C)   PURCHASE PRICE PER STOCKHOLDER SHARE.  The purchase price to be
paid per Stockholder Share upon the exercise of the Put Option or the Call
Option ("Purchase Price") shall be determined as follows:


       (1)   If the Put Option or the Call Option is exercised at any time on or
after January 11, 1998 but before July 11, 1998, the purchase price per
Stockholder Share shall be $871.93; and


       (2)   If the Put Option or the Call Option is exercised at any time on or
after July 11, 1998 but before July 11, 1999, the purchase price per Stockholder
Share shall be $980.92.


       (D)   EXERCISE OF PUT OPTION OR CALL OPTION; CLOSING DATE.  The party
exercising the Put Option or the Call Option, as the case may be, shall deliver
written notice of its exercise (an





                                       5
<PAGE>   6

"EXERCISE NOTICE"), designating a place for the closing in the greater Boston
area and the Closing Date for such transaction, which Closing Date shall be not
more than sixty (60) days after delivery of such Exercise Notice.


       (E)   PAYMENT OF PURCHASE PRICE.  At ImmunoGen's option, ImmunoGen shall
pay the Purchase Price either (i) by certified or bank check, or (ii) by the
delivery to the Stockholders of shares of ImmunoGen's common stock (the
"Exchanged Shares"). If ImmunoGen opts to pay the Purchase Price by delivery of
Exchanged Shares, the number of Exchanged Shares ImmunoGen shall be obligated to
deliver to each Stockholder at the closing shall be determined by dividing the
Purchase Price due each Stockholder by the average closing price for shares of
ImmunoGen's common stock as reported in the NASDAQ National Market System during
the twenty (20) trading days preceding the Closing Date. (Such average closing
price shall be determined by adding the closing prices for each of such twenty
(20) days and dividing such sum by twenty (20).) ImmunoGen shall not be
obligated to exchange any fractional shares, and to the extent any Stockholder
is due an amount exceeding the value of a whole number of shares of ImmunoGen
common stock, then ImmunoGen shall pay such Stockholder the difference by check.

       SECTION 5.   BOARD OF DIRECTORS.


       The parties hereto agree that within two (2) years of the date of this
Agreement they will cause to be elected to the Board of Directors of ATI one
director who is not an employee of either ImmunoGen or DFCI.

       SECTION 6.   GENERAL.


       (A)   DELIVERY OF STOCK CERTIFICATES.  At any closing hereunder, the
seller of Stockholder Shares and/or Additional Stockholder Shares shall be
required to deliver certificates for the Stockholder Shares and/or Additional
Stockholder Shares being sold to ImmunoGen with appropriate endorsement or with
separate stock powers and such other documentation as is necessary to vest
ImmunoGen with absolute title to the Stockholder Shares and/or Additional
Stockholder Shares being sold, free and clear of all liens, encumbrances,
charges, liabilities, debts and restrictions; and ImmunoGen shall deliver to the
Stockholder the consideration determined in accordance with this Agreement for
such shares.


       (B)   ADJUSTMENT PROVISIONS.  If ATI shall pay a stock dividend or
declare a stock split on or with respect to any of





                                       6
<PAGE>   7

its Common Stock, or otherwise distribute securities of ATI to the holders of
its Common Stock, the number of shares of stock or other securities of ATI
issued with respect to the Stockholder Shares then subject to the Put Option and
the Call Option shall be added to the Stockholder Shares then subject to the Put
Option and the Call Option without any change in the aggregate purchase price
otherwise payable with respect thereto. If ATI shall distribute to its
stockholders shares of stock of another corporation, the shares of stock of such
other corporation distributed with respect to the Stockholder Shares then
subject to the Put Option and the Call Option shall be added to the Stockholder
Shares covered by the Put Option and the Call Option without any change in the
aggregate purchase price otherwise payable with respect thereto.


       If the outstanding shares of Common Stock of ATI shall be subdivided into
a greater number of shares or combined into a smaller number of shares, or in
the event of a reclassification of the outstanding shares of Common Stock of
ATI, or if ATI shall be a party to any capital reorganization, there shall be
substituted for the Stockholder Shares then covered by the Put Option and the
Call Option such amount and kind of securities as are issued in such
subdivision, combination, reclassification, or capital reorganization in respect
of the Stockholder Shares subject to the Put Option and the Call Option
immediately prior thereto, without any change in the aggregate purchase price
otherwise payable with respect thereto.


       SECTION 7.   MISCELLANEOUS.


       (A)   ENTIRE AGREEMENT.  This Agreement represents the entire contract
among the parties relating to the subject matter hereof. No statements,
representations, warranties (express or implied), writings, understandings or
agreements of any party or of any representative of any party which are not set
forth herein relating to such subject matter shall be binding. This Agreement
may not be amended, modified or altered, except by a supplemental writing
executed by the parties hereto which expressly refers to this Agreement.


       (B)   BINDING EFFECT.  This Agreement shall be binding upon and inure to
the benefit of the parties, their transferees, successors, assigns, estates,
personal and legal representatives, executors, administrators and heirs,
provided that if ImmunoGen shall purchase any shares from any Stockholder,
ImmunoGen shall not succeed to such Stockholder's rights or obligations with
regard to such shares.


       (C)   GOVERNING LAW.  This Agreement shall be governed by the law of the
Commonwealth of Massachusetts.




                                       7
<PAGE>   8

       (D)   NOTICES.  Any notice required hereunder shall be deemed to have
been validly given, if sent by certified mail, return receipt requested, postage
prepaid or delivered in hand, or by recognized delivery service providing
evidence of delivery, in the case of ATI, at the following address to the
attention of its President: 148 Sidney Street, Cambridge, MA 02139, in the case
of ImmunoGen at the following address to the attention of its President: 148
Sidney Street, Cambridge, MA 02139, in the case of DFCI, at the following
address to the attention of its President: 44 Binney Street, Boston,
Massachusetts 02115, and in the case of Schlossman, at the following address:
One Fox Place, Newton Center, Massachusetts 02159, or in any case to such other
address as may be furnished in writing by one party to the other parties hereto.


       (E)   COUNTERPARTS.  This Agreement may be executed in counterparts, all
of which together shall constitute one and the same instrument.


       (F)   WAIVER.  No express or implied waiver by any party of any default
shall be a waiver of any future or subsequent default. The failure or delay of
any party in exercising any rights granted hereunder shall not constitute a
waiver of any such right and any single or partial exercise of any particular
right by any party shall not exhaust the same or constitute a waiver of any
other right hereunder.


       (G)   CAPTIONS.  All section and descriptive headings are inserted for
convenience only, and shall not affect any construction or interpretation
hereof.


       (H)   SEVERABILITY.  If any provision hereof should be held invalid,
illegal or unenforceable in any respect in any jurisdiction, then, to the
fullest extent permitted by law, (a) all other provisions hereof shall remain in
full force and effect in such jurisdiction and shall be liberally construed in
order to carry out the intentions of the parties hereto as nearly as may be
possible and (b) such invalidity, illegality or unenforceability shall not
affect the validity, legality or enforceability of such provision in any other
jurisdiction. To the extent permitted by applicable law, the parties hereto
hereby waive any provision of law that would render any provision hereof
prohibited or unenforceable in any respect.


       (I)   FURTHER INSTRUMENTS AND ASSURANCES.  Each of the parties hereto
agrees to duly execute and deliver, or cause to be duly executed and delivered,
such further instruments and do and cause to be done such further acts and
things, including, without limitation, the filing of such additional
assignments, agreements, documents and instruments, that may be necessary or





                                       8
<PAGE>   9

as the other party hereto may at any time and from time to time reasonably
request in connection with this Agreement or to carry out more effectively the
provisions and purposes of, or to better assure and confirm unto such other
party its rights and remedies under, this Agreement.



IN WITNESS WHEREOF, this Agreement has been executed as a sealed instrument by
the individual parties and the corporate parties by their authorized officers,
as the act and deed of said parties, on the day and year first hereinabove
written.


                                            IMMUNOGEN, INC.



                                            By: /s/ Frank Pocher
                                                -------------------------------
                                            Name: Frank Pocher
                                            Title: Vice President


                                            APOPTOSIS TECHNOLOGY, INC.



                                            By: /s/ Mitchel Sayare
                                                -------------------------------
                                            Name: Mitchel Sayare
                                            Title: President





THE STOCKHOLDERS:


                                            DANA-FARBER CANCER INSTITUTE, INC.

                                            By: /s/ Bernard W. Janicki, Ph.D.
                                                -------------------------------
                                            Name: Bernard W. Janicki, Ph.D.
                                            Title: Director for Research




                                            Dr. Stuart Schlossman

                                            /s/ Stuart F. Schlossman
                                            -----------------------------------
                                            Dr. Stuart Schlossman




                                       9


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