IMMUNOGEN INC
S-3, 1997-07-22
PHARMACEUTICAL PREPARATIONS
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<PAGE>   1
   
    AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JULY 22, 1997
    

                               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 (617) 769-4242
      (Address, including zip code, and telephone, including area code, of
                    registrant's principal executive offices)

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

                                 MITCHEL SAYARE
                              CHAIRMAN OF THE BOARD
                                 IMMUNOGEN, INC.
                             333 Providence Highway
                                Norwood, MA 02062
                                 (617) 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

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

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


   


<TABLE>

                            CALCULATION OF REGISTRATION FEE
<CAPTION>
                                        PROPOSED     PROPOSED
                                        MAXIMUM      MAXIMUM         
TITLE OF EACH                           OFFERING     AGGREGATE    AMOUNT OF
CLASS OF SECURITIES     AMOUNT TO BE    PRICE PER    OFFERING     REGISTRATION
TO BE REGISTERED        REGISTERED      SHARE        PRICE        FEE
                                                                  
<S>                      <C>             <C>         <C>            <C>
Common Stock, par                                                 
value $.01 per share     2,757,862       $1.27(1)    $3,502,485     $1,051

</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 as reported on the
        Nasdaq National Market on July 18, 1997.

    



<PAGE>   2

         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. / /


<PAGE>   3


                                  PROSPECTUS
                                      
   
                               IMMUNOGEN, INC.
                       2,757,862 SHARES OF COMMON STOCK
                        (PAR VALUE OF $.01 PER SHARE)
    

   
The 2,757,862 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 Stockholders 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 July 18,
1997, the closing sale price of the Common Stock, as reported by Nasdaq, was
$1.34 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      , 1997.
    

<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
(617) 769-4242 and directed to the attention of the Treasurer.


                                      - 2 -



<PAGE>   5



                                TABLE OF CONTENTS


                                                                           PAGE
                                                                           ----

RISK FACTORS                                                               -4-
THE COMPANY                                                                -9-
SELLING STOCKHOLDERS                                                       -9-
PLAN OF DISTRIBUTION                                                       -10-
LEGALITY OF COMMON STOCK                                                   -11-
EXPERTS                                                                    -11-
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE                          -11-
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, 1996 (the "Form
10-K") (as amended by Amendment No.1 on Form 10-K/A dated April 7, 1997), the
Quarterly Reports on Form 10-Q for the quarters ended September 30, 1996 (as
amended by Amendment No.1 on Form 10-Q/A dated April 7, 1997), December 31, 1996
(as amended by Amendment No.1 and Amendment No.2 on Form 10-Q/A dated April 7,
1997 and June 5, 1997, respectively) and March 31, 1997, and under "Business" in
the Form 10-K/A, 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 July 1, 1997 were approximately $2.0 million. This amount
includes $1.0 million received in June 1997 pursuant to a private placement of
Series D convertible preferred stock (the "Series D Stock") to an institutional
investor (the "Series D Financing"). The Company anticipates that its existing
capital resources, including the $1.0 million proceeds of the Series D
Financing, will enable it to maintain its current and planned operations
through approximately  August, 1997. The Series D financing represents a
third investment pursuant to a financing agreement entered into by the
Company in October 1996 (the "October 1996 Private Placement"). The October
1996 Private Placement granted the Company the right to require the investor to
purchase up to $12.0 million of convertible preferred stock from the Company in
a series of private placements. To date, the Company has received an aggregate
of $7.0 million pursuant to the agreement. Because minimum stock price and
other requirements have not been maintained, the investor was not obligated to
purchase the Series D Stock and is no longer obligated to fund the remaining
$5.0 million which had been available to the Company under this agreement.
However, in discussions with the Company, the investor has indicated a
willingness to make additional  investments in the Company on an ongoing basis
to the extent necessary to fund  the Company's operations, subject to certain
conditions to be agreed upon.  Pursuant to those discussions, the Company may
issue an additional $1.0 million of its preferred stock to the investor in the
near term, if certain short-term  financing objectives are met. In addition,
the Company is actively engaged in  discussions regarding potential financing
and/or strategic partnering  arrangements involving an equity investment.
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 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, if 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 ("FDA") regulations applicable to such testing can result in
delay, suspension or cancellation of such testing, and/or refusal by FDA to
accept the results of such testing. In addition, 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, 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 show any current or future product candidate to
be safe and effective or the data derived therefrom will be suitable for
submission to 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 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. 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 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.


                                      - 5 -
<PAGE>   8



There has been significant litigation in the biotechnology industry regarding
patent and other intellectual property rights and 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 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 owns three issued United States patents. It has also applied for
several patents. In addition, Dana-Farber Cancer Institute ("Dana-Farber") has
filed applications for a number of patents to which the Company has exclusive
rights, and several of these have been issued as patents. There can be no
assurance that any patent applications will issue as patents or that any issued
patents will provide the Company with significant protection against
competitors. The Company's license agreement with Dana-Farber requires ImmunoGen
to use all reasonable efforts, consistent with sound and reasonable business
practices and judgment, to effect introduction of licensed products into the
commercial market as soon as practicable. Failure to do so can result in the
loss of the Company's exclusive rights to such licensed products.

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, and 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.




                                      - 6 -

<PAGE>   9




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.

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 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.

PROPOSED INTERNATIONAL TREATY. More than 150 nations, including the United
States, are signatories to an international treaty restricting the manufacture
and sale of chemical substances identified therein as components of chemical
warfare. Ricin, a natural toxin obtained from a cultivated plant, is among the
substances restricted pursuant to the proposed treaty. Ricin is used by the     
Company in one of its two technology platforms, including Oncolysin B. If the
treaty is ratified by the United States, such action may adversely impact the
availability and price of ricin.

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 it 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.



                                      - 7 -


<PAGE>   10

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 21,779,767
shares of Common Stock were freely tradable on the open market as of July 8,
1997. In addition, approximately 874,270 shares are eligible for sale pursuant
to Rule 144 of the Securities Act. Also, as of July 8, 1997, options to
purchase 1,500,766 shares of Common Stock were outstanding pursuant to the
Company's stock option plans and 1,282,129 of such shares were vested and could
be exercised at any time prior to their respective expiration dates. As of July
8, 1997, 26,738 shares of Common Stock were issuable upon the exercise of
warrants issued in connection with a capital lease financing in March 1994,
1,923,372 shares of Common Stock were issuable upon the exercise of warrants
issued to date in connection with the Company's March 1996 Financing and
1,674,754 shares of Common Stock were issuable upon the exercise of warrants
issued to date in connection with the Company's October 1996 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 private placement, 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. These warrants expire in March
2001. In June 1996, a second $2.5 million convertible debenture (the "Second
Debenture") was issued and then converted into Series A Convertible Preferred
Stock ("Series A Stock") in October 1996. Each share of Series A Stock is
convertible at any time into a number of shares of Common Stock determined by
dividing $1,000 by the lower of (i) $2.50 or (ii) 85% of the average of the
closing bid price for the five days prior to conversion (the "Conversion Date
Price"). As of July 8, 1997, 1,400 of the 2,500 shares of Series A Stock plus
accrued interest thereon had been converted into 1,328,744 shares of the
Company's Common Stock. In June 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. These 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
receives 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 will be exercisable at $4.00 per share
and expire five years after the date of issuance. As of July 8, 1997, warrants
to purchase 664,372 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 the
approximately 3,627,000 shares of Common Stock which are issued or issuable
upon conversion of the Series A Stock, the First and Second Warrants, the
Additional Warrant and the warrants issued as a finder's fee. If the Series A
Stock and all related warrants become convertible into more than 3,627,000
shares, the Company will be obligated to register additional shares of Common
Stock.

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 July 8, 1997, all of the 3,000 shares of the Series B Stock
plus


                                      - 8 -

<PAGE>   11

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
(the "January 1997 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 the approximately 1,916,666 shares of Common
Stock which are issued or issuable upon conversion of the Series B Stock and
October 1996 Warrants. If the Series B Stock and October 1996 Warrants become
convertible into more than 1,916,666 shares, the Company will be obligated to
register additional shares of Common Stock.

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. Each
share of Series C Stock is convertible at any time into a number of shares of
Common Stock determined by dividing $1,000 by the lower of (i) $2.61 or (ii)
85% of the average closing bid price for the five days prior to conversion (the
"Conversion Date Price"). As of July 8, 1997, 2,300 of the 3,000 shares of the
Series C Stock plus accrued dividends thereon had been converted into 2,018,558
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 the approximately
6,000,000 shares of Common Stock which are issued or issuable upon conversion
of the Series C Stock and April 1997 Warrants. If the Series C Stock and April
1997 Warrants become convertible into more than 6,000,000 shares, the Company
will be obligated to register additional shares of Common Stock.

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, among others, 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.

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. The Series A Stock, Series C Stock and Series D Stock can
be converted into shares of the Company's Common Stock at any time at a discount
to the then market price of the Company's Common Stock. See "Shares Eligible for
Future Sale."

                                   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, Apoptosis Technology, Inc. ("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 (617) 769-4242.

   
                               SELLING STOCKHOLDERS

The shares of Common Stock offered hereby by Southbrook International
Investments, Ltd. ("Southbrook") are issuable upon conversion of 1,000 shares of
the Company's Series D Stock issued to Southbrook in the Series D Financing and
exercise of an aggregate of 1,147,754 April 1997 Warrants (issued in connection
with the Series C Stock), of which 860,815 are held by Southbrook and 286,939
are held by Brown Simpson, LLC. The shares of Common Stock offered hereby by
Stuart Schlossman, M.D. were issued by the Company to Dr. Schlossman as
repayment of certain sums owed by the Company to Dr. Schlossman. 

    

                                      - 9 -

<PAGE>   12
   
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 
number of shares of Common Stock that will ultimately be issued to Southbrook 
upon conversion of the Series D Stock is dependent upon a conversion formula 
which relies in part on the closing bid price of the Common Stock for the 
five trading days immediately preceding the date of conversion and therefore 
cannot be determined at this time.

<TABLE>

The following table sets forth information with respect to the beneficial
ownership of the Company's Common Stock by the Selling Stockholder as of July 
8, 1997, and as adjusted to reflect the sale of the Common Stock offered
hereby by the Selling Stockholders.

<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>
Southbrook International    2,910,297(2)(4)       2,429,442       1,374,354(3)     6.3(4)
Investments, Ltd.

Brown Simpson, LLC            286,939               286,939        0                *

Stuart Schlossman, M.D.        41,481(5)             41,481        0                *


</TABLE>

* Less than 1%

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

(2) Includes shares of Common Stock issuable upon conversion of 1,000 shares of
Series D Stock, 700 shares of Series C Stock, shares of Common Stock issuable
upon exercise of the October 1996 Warrants, the January 1997 Warrants and the
April 1997 Warrants and shares owned by the Selling Stockholder at July 8, 1997.

(3) Includes shares of Common Stock issuable upon conversion of 700 shares of
Series C Stock and exercise of the October 1996 Warrants and the January 1997
Warrants, and shares owned by Southbrook at July 8, 1997.

(4) Based on 21,779,767 shares of Common Stock outstanding on July 8, 1997.
Certain of these shares are covered by the Company's registration statement on
Form S-3, registration number 333-22153. The Convertible Preferred Stock
Purchase Agreement limits the conversion right of Southbrook such that the
maximum number of shares of the Company's Common Stock issued upon conversion
or exercise of the preferred stock and warrants may not exceed 4.9% of the then
issued and outstanding shares of the Company's Common Stock following such
conversion.

(5) Based on 21,779,767 shares of Common Stock outstanding on July 8, 1997.


                              PLAN OF DISTRIBUTION

The 2,757,862 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.
    

                                     - 10 -
<PAGE>   13

The Company has agreed to use its best efforts to maintain the effectiveness of
the registration of the shares being offered hereunder until June 27, 2001 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. Southbrook has agreed not to sell a net number
of shares of Common Stock (through short positions or otherwise) in excess of
the greater of (i) 20% of the average trading volume for the week preceding the
sale or (ii) on any given day, 20% of the trading volume on that day.

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 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, 1996, 
as amended by Amendment No.1 on Form 10-K/A dated April 7, 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.

                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,
1996, as amended by Amendment No. 1 on Form 10-K/A dated April 7, 1997.

(b) The Company's Quarterly Report on Form 10-Q for the quarter ended September
30, 1996, as amended by Amendment No. 1 dated April 7, 1997.

(c) The Company's Quarterly Report on Form 10-Q for the quarter ended December
31, 1996, as amended by Amendment No. 1 and Amendment No. 2 on Form 10-Q/A 
dated April 7, 1997 and June 5, 1997, respectively.

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

(e) The Company's Current Report on Form 8-K for the July 15, 1996 event.

(f) The Company's Current Report on Form 8-K for the March 18, 1997 event.

(g) The Company's Current Report on Form 8-K for the April 15, 1997 event.

(h) 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.





                                     - 11 -


<PAGE>   14




                 PART II. INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

<TABLE>
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.
<CAPTION>

                     <S>                           <C>
                     SEC Registration Fee          $1,051.00

                     Legal Fees and Expenses        1,500.00

                     Accounting Fees and Expenses   1,000.00

                     Miscellaneous                    500.00
                                                   ---------
         TOTAL                                     $4,051.00
                                                   =========
</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.


                                      II-1


<PAGE>   15



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.

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.

In addition, the Registration Rights Agreement, filed as Exhibit 99.2 hereto,
contains provisions for indemnification by the Selling Shareholder of the
Registrant and its officers, directors, and controlling persons against certain
liabilities under the Securities Act.

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 (previously filed)

              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  Convertible Preferred Stock Purchase Agreement dated as of
                    October 16, 1996 between Southbrook International
                    Investments, Ltd. and the Registrant (previously filed as
                    Exhibit No. 10.34 to the Registrant's Quarterly Report on
                    Form 10-Q for the period ended September 30, 1996 (File No.
                    0-17999), and incorporated herein by reference)

              99.2  Registration Rights Agreement dated as of October 16, 1996
                    between Southbrook International Investments, Ltd. and the
                    Registrant (previously filed as Exhibit No. 10.35 to the
                    Registrant's Quarterly Report on Form 10-Q for the period
                    ended September 30, 1996 (File No. 0-17999), and
                    incorporated herein by reference)

              99.3  Form of Warrant issued to Southbrook International 
                    Investments, Ltd. (previously filed as Exhibit No. 10.38 to
                    the Registrant's Quarterly Report on Form 10-Q for the
                    period ended December 31, 1996 (File No. 0-17999),  and
                    incorporated herein by reference)
        









                                      II-2



<PAGE>   16


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)(sec.230.424(b) of this
chapter) 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;

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>   17


                                   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 Cambridge, Massachusetts on July 22, 1997.

                                    IMMUNOGEN, INC.

                                    By: /s/ Mitchel Sayare
                                        ----------------------------------------
                                        Mitchel Sayare
                                        Chairman Of The Board and
                                        Chief Executive Officer

                                POWER OF ATTORNEY
                                -----------------

<TABLE>

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.


<CAPTION>

         Signature                       Title                                Date
         ---------                       -----                                ----


<S>                          <C>                                        <C>
/s/ Mitchel Sayare           Chairman of the Board of                   July 22, 1997
- --------------------------    Directors and Chief
Mitchel Sayare                Executive Officer
                              (principal executive officer)


/s/ Kathleen A. Carroll      Vice President, Finance and                July 22, 1997
- --------------------------    Administration, Treasurer
Kathleen A. Carroll           (principal financial officer and
                              principal accounting officer)


/s/ Walter A. Blattler       Executive  Vice President,                 July 22, 1997
- --------------------------    Science and Technology and Director
Walter A. Blattler            
                       

                             Director
- --------------------------
Michael R Eisenson                          
</TABLE>


                                      II-4


        
<PAGE>   18


/s/ Stuart F. Feiner         Director                        July 22, 1997
- --------------------------                                   
Stuart F. Feiner                                             
                                                        
                             Director                   
- --------------------------                                   
Donald E. O'Neill                                            

/s/ David W. Carter          Director                        July 22, 1997
- --------------------------
David W. Carter              

                                                             








                                      II-5









<PAGE>   1
                                                                      EXHIBIT 5


      [MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C. LETTERHEAD]


                                              July 22, 1997



ImmunoGen, Inc.
333 Providence Highway
Norwood, Massachusetts 02062


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 2,757,862 shares (the
"Shares") of its common stock, $0.01 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 certificated 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 and validly authorized by the Company and (ii) the Shares, when
sold, will have been duly and 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.

        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.












<PAGE>   2

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


ImmunoGen, Inc.
July 22, 1997
Page 2


                                                Very truly yours,


                                                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 2,757,862 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 August 28, 1996, except for Note K as to which the date is April 4, 1997,
on our audits of the consolidated financial statements of ImmunoGen, Inc. as of
June 30, 1995 and 1996, and for the three years ended June 30, 1994, 1995 and
1996, which report is included in the Company's 1996 Annual Report on Form 10-K,
as amended by Amendment No. 1 on Form 10-K/A dated April 7, 1997.

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

                                                     COOPERS & LYBRAND L.L.P.

Boston, Massachusetts
July 22, 1997



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