<PAGE>   1



                                                                    EXHIBIT 10.2



                                  AMENDMENT TO
                           1994 DIRECTORS' EQUITY PLAN
                                       OF
                           BIOTRANSPLANT INCORPORATED

         The 1994 Directors' Equity Plan, as amended (the "Plan") be and hereby
is amended as follows:

         1.       The number 60,000 in the second line of Section 4(a) of the
                  Plan shall be deleted and the number 50,000 shall be inserted
                  in lieu thereof.


                         Adopted by the Board of Directors on February 24, 1997 
                         Approved by the Stockholders on May 20, 1997



















<PAGE>   2



                               BIOTRANSPLANT, INC.

                           1994 DIRECTORS' EQUITY PLAN


1.       PURPOSE

         The purpose of this 1994 Directors' Equity Plan (the "Plan") of
BioTransplant, Inc. (the "Company") is to promote the recruiting and retention
of highly qualified outside directors and to strengthen the commonality of
interest between directors and stockholders. Except where the context otherwise
requires, the term "Company" shall include all subsidiaries of the Company as
defined in Sections 424(e) and 424(f) of the International Revenue Code of 1986,
as amended or replaced from time to time (the "Code").

2.       ADMINISTRATION

         The Plan will be administered by the Compensation Committee of the
Board of Directors of the Company or by the Board of Directors of the Company as
a whole, whose construction and interpretation of the terms and provisions of
the Plan shall be final and conclusive. Reference herein to the Board of
Directors relating to the administration and interpretation of the Plan shall
mean the Compensation Committee or the Board of Directors, as the case may be.
Grants of stock options ("Options") and restricted stock awards ("Awards") under
the Plan and the amount and nature of the Options and Awards to be granted shall
be automatic and non-discretionary in accordance with Sections 5 and 6. However,
all questions of interpretation of the Plan or of any Options or Awards issued
under it shall be determined by the Board of Directors and such determination
shall be final and binding upon all persons having an interest in the Plan. No
director shall be liable for any action or determination under the Plan made in
good faith.

3.       PARTICIPATION IN THE PLAN

         Only directors of the Company who are not employees of the Company
shall be eligible to be granted Options and Awards under the Plan. Directors who
are employed by, or affiliates of, either HealthCare Investment Corporation,
Sandoz Pharma Ltd. or Charles River Laboratories, Inc. or any of their
respective related entities, are ineligible to receive Options or Awards under
the Plan. A director may notify the Company in writing that he or she
irrevocably elects not to participate in the Plan and such director shall
henceforth be deemed to be ineligible.




<PAGE>   3



4.       STOCK SUBJECT TO THE PLAN

         (a) The maximum number of shares which may be issued under the Plan
shall be 60,000 shares of the Company's Common Stock, $.01 par value per share
("Common Stock"), subject to subsequent adjustment as provided in Section 10.

         (b) If any outstanding Option under the Plan for any reason expires or
is terminated without having been exercised in full, or any Common Stock issued
pursuant to an Award shall be repurchased by the Company pursuant to the terms
of such Award, the shares allocable to the unexercised portion of such Option
and/or such repurchased shares shall again become available for grant pursuant
to the Plan.

         (c) All Options granted under the Plan shall be non-statutory options
which are not intended to meet the requirements of Section 422 of the Code.

5.       TERMS, CONDITIONS AND FORMS OF OPTIONS

         Each Option granted under the Plan shall be evidenced by a written
agreement in substantially the form of EXHIBIT A hereto or in such other form as
the Board of Directors shall from time to time approve, which agreements shall
comply with and be subject to the following terms and conditions:

         (a) OPTION GRANT DATES. Each eligible director shall automatically be
granted an Option to purchase 12,500 shares of Common Stock upon the later of
(i) his or her initial election as a director, or (ii) the adoption by the Board
of Directors of the Plan.

         (b) OPTION EXERCISE PRICE. The option exercise price per share for each
Option granted under the Plan shall equal (i) the last reported sales price per
share of the Company's Common Stock on the Nasdaq National Market (or, if the
Company is traded on a nationally recognized securities exchange on the date of
the grant, the reported exchange) on the date of grant (or if no such price is
reported on such date, such price as reported on the nearest preceding date on
which such price is reported) or (ii) if the Common Stock is not traded on
Nasdaq or an exchange, the fair market value per share on the date of grant as
determined by the Board of Directors.

         (c) OPTIONS NON-TRANSFERABLE. Each Option granted under the Plan by its
terms shall not be transferable by the optionee otherwise than by will or by the
laws of descent and distribution, or pursuant to a qualified domestic relations
order (as defined in Section 414(p) of the Code) and shall be exercised during
the lifetime of the optionee only by such optionee.

         (d) EXERCISE PERIOD. Each Option granted under the Plan may be
exercised on a cumulative basis as to 20% of the shares on the first anniversary
of the date of

                                       -2-

<PAGE>   4



grant and an additional 20% at the end of each one-year period thereafter. Each
Option granted under the Plan, to the extent it has vested, may be exercised in
whole or in part at any time prior to the tenth anniversary of the date of
grant, but in no event may it be exercised thereafter.

         (e) EXERCISE PROCEDURE. Options may be exercised only by written notice
to the Company at its principal office accompanied by payment of the full
consideration for the shares as to which they are exercised.

         (f) PAYMENT OF PURCHASE PRICE. Payment of the exercise price shall be
made (i) by delivery of cash or a check to the order of the Company in an amount
equal to the exercise price, (ii) by delivery to the Company of shares of Common
Stock of the Company already owned and held by the optionee for at least twelve
months and having a fair market value equal in amount to the exercise price of
the Option being exercised, or (iii) by any combination of such methods of
payment. The fair market value of any shares of Common Stock which may be
delivered upon exercise of an Option shall be determined by the Board of
Directors of the Company as of the date that such shares are delivered.

6.       TERMS, CONDITIONS AND FORMS OF RESTRICTED STOCK AWARDS

         Each Award granted under the Plan shall be evidenced by a written
agreement in substantially the form of EXHIBIT B hereto or in such other form as
the Board of Directors shall from time to time approve, which agreements shall
comply with and be subject to the following terms and conditions:

         (a) AWARD GRANT DATES. Each eligible director shall automatically be
granted an Award to purchase 2,500 shares of Common Stock upon the later of (i)
his or her initial election as a director, or (ii) the adoption by the Board of
Directors of the Plan.

         (b) AWARD PRICE. The price per share of Common Stock subject to each
Award shall be $.01 per share, payable by cash or check upon execution of the
written agreement evidencing the Award.

         (c) VESTING PERIOD. Each Award granted under the Plan shall vest during
the period in which an eligible director continues to serve as a director of the
Company, at the rate of 25% of the shares on the first anniversary of the date
of grant and an additional 25% at the end of each one-year period thereafter.

         (d) REPURCHASE RIGHT. The Company shall have the right to repurchase
any or all shares of Common Stock subject to an Award which shall not have
become vested as of the date an eligible director ceases to serve as a director
at a repurchase price of $.01 per share, such right to be exercised within 90
days after such director

                                       -3-

<PAGE>   5



ceases to serve as a director of the Company. The Company shall have the right
to assign or transfer all or part of such repurchase rights to one or more third
parties, including other stockholders of the Company.

7.       ASSIGNMENTS

         The rights and benefits under the Plan may not be assigned, except as
provided in Section 5.

8.       TIME FOR GRANTING OPTIONS AND AWARDS

         All Options and Awards subject to the Plan shall be granted, if at all,
not later than six (6) years after the date of the Board's adoption of the Plan.

9.       LIMITATION OF RIGHTS

         (a) NO RIGHT TO CONTINUE AS A DIRECTOR. Neither the Plan, nor the
granting of an Option or Award nor any other action taken pursuant to the Plan,
shall constitute or be evidence of any agreement or understanding, express or
implied, that the Company will retain a director for any period of time.

         (b) NO STOCKHOLDER RIGHTS FOR OPTIONS. An optionee shall have no rights
as a stockholder with respect to the shares covered by his or her Option under
the date of the issuance to him or her of a stock certificate thereof, and no
adjustment will be made for dividends or other rights for which the record date
is prior to the date such certificate is issued.

10.      ADJUSTMENT PROVISIONS RELATING TO OPTIONS

         (a) RECAPITALIZATIONS. If, after the Effective Date of the Plan,
through or as a result of any recapitalization, reclassification, stock
dividend, stock split, reverse stock split or other similar transaction, (i) the
outstanding shares of Common Stock are increased or decreased or are exchanged
for a different number or kind of shares or other securities of the Company, or
(ii) additional shares or new or different shares or other securities of the
Company or other non-cash assets are distributed with respect to such shares of
Common Stock or other securities, an appropriate and proportionate adjustment
shall be made in (x) the maximum number and kind of shares reserved for issuance
under the Plan, (y) the number and kind of shares or other securities subject to
then outstanding Options under the Plan, and (z) the price for each share
subject to any then outstanding Options under the Plan, without changing the
aggregate purchase price as to which such Options remain exercisable, provided
that no adjustment shall be made pursuant to this Section 10 if such adjustment
would cause the Plan to fail to comply with Rule 16b-3 under the Securities
Exchange Act of 1934, (the "Exchange Act") as amended or any successor rule
("Rule 16b-3").

                                       -4-

<PAGE>   6




         (b) REORGANIZATIONS. After the Effective Date of the Plan, in the event
of a consolidation or merger or sale of all or substantially all of the assets
of the Company in which outstanding shares of Common Stock are exchanged for
securities, cash or other property of any other corporation or business entity
or in the event of a liquidation of the Company, the Board of Directors of the
Company, or the board of directors of any corporation assuming the obligations
of the Company, may, in its discretion, take any one or more of the following
actions, as to the outstanding Options: (i) provide that such Options shall be
assumed, or equivalent options shall be substituted, by the acquiring or
succeeding corporation (or an affiliate thereof), (ii) upon written notice to
the optionees, provide that all unexercised Options will terminate immediately
prior to the consummation of such transaction unless exercised by the optionee
within a specified period following the date of such notice, (iii) in the event
of a merger under the terms of which holders of the Common Stock of the Company
will receive upon consummation thereof a cash payment for each share surrendered
in the merger (the "Merger Price"), make or provide for a cash payment to the
optionees equal to the difference between (A) the Merger Price times the number
of shares of Common Stock subject to such outstanding Options (to the extent
then exercisable at prices not in excess of the Merger Price) and (B) the
aggregate exercise price of all such outstanding Options in exchange for the
termination of such options, and (iv) provide that all outstanding Options shall
become exercisable in full.

11.      AMENDMENT OF THE PLAN

         (a) The provisions of Sections 3, 5(a), 5(b) and 6 of the Plan shall
not be amended more than once every six months, other than to comport with
changes in the Code, the Employee Retirement Income Security Act of 1974, as
amended, or the rules thereunder. Subject to the foregoing, the Board of
Directors may at any time, and from time to time, modify or amend the Plan in
any respect, except that if at any time the approval of the stockholders of the
Company is required as to such modification or amendment under Rule 16b-3. The
Board of Directors may not effect such modification or amendment without such
approval.

         (b) The termination or any modification or amendment of the Plan shall
not, without the consent of an eligible director, affect his or her rights under
an Option or Award previously granted to him or her. With the consent of each
eligible director affected, the Board of Directors may amend outstanding option
agreements or restricted stock award agreements in a manner not inconsistent
with the Plan. The Board of Directors shall have the right to amend or modify
the terms and provisions of the Plan or any outstanding Option or Award to the
extent necessary to ensure the qualification of the Plan under Rule 16b-3.



                                       -5-

<PAGE>   7



12.      WITHHOLDING

         The Company shall have the right to deduct from payments of any kind
otherwise due to an eligible director any federal, state or local taxes of any
kind required by law to be withheld with respect to any shares issued pursuant
to Awards or upon exercise of Options under the Plan.

13.      NOTICE

         Any written notice to the Company required by any of the provisions of
the Plan shall be addressed to the Treasurer of the Company and shall become
effective when it is received by the Company.

14.      EFFECTIVE DATE AND DURATION OF THE PLAN

         (a) EFFECTIVE DATE. The Plan shall become effective on June 27, 1994,
but no Option granted under the Plan shall become exercisable nor any Award
become vested unless and until the Plan shall have been approved by the
Company's stockholders. If such stockholder approval is not obtained within
twelve months after the date of the Board's adoption of the Plan, (i) all
Options granted under the Plan shall terminate and no further Options shall be
granted under the Plan, and (ii) all Awards shall be cancelled and no further
Awards shall be granted under the Plan. Amendments to the Plan not requiring
stockholder approval shall become effective when adopted by the Board of
Directors; amendments requiring stockholder approval (as provided in Section
11(a)) shall become effective when adopted by the Board of Directors, but no
Option issued after the date of such amendment shall become exercisable nor
shall any Award vest (to the extent that such amendment to the Plan was required
to enable the Company to grant such Option or Award to a particular eligible
director) unless and until such amendment shall have been approved by the
Company's stockholders. If such stockholder approval is not obtained within
twelve months of the Board's adoption of such amendment, any Options granted on
or after the date of such amendment shall terminate, and shares issued pursuant
to an Award shall be cancelled, in each case to the extent that such amendment
to the Plan was required to enable the Company to grant such Option or Award to
a particular eligible director. Subject to this limitation, Options and Awards
may be granted under the Plan at any time after the effective date and before
the date fixed for termination of the Plan.

         (b) TERMINATION. Unless earlier terminated pursuant to Section 10, the
Plan shall terminate upon the earlier of (i) June 27, 2000, or (ii) the date on
which all shares available for issuance under the Plan shall have been issued
pursuant to the grant of Awards and exercise of Options granted under the Plan.
If the date of termination is determined under (i) above, then Options and
Awards outstanding on

                                       -6-

<PAGE>   8



such date shall continue to have force and effect in accordance with the
provisions of the instruments evidencing such Options and awards.

15.      GENERAL REQUIREMENTS

         (a) INVESTMENT REPRESENTATIONS. The Company may require any person to
whom an Option or Award is granted, as a condition of exercising such Option or
Award, to give written assurance in substance and form satisfactory to the
Company to the effect that such person is acquiring the Common Stock subject to
the Option or Award for his or her own account for investment and not with any
present intention of selling or otherwise distributing the same, and to such
other effects as the Company deems necessary or appropriate in order to comply
with federal and applicable state securities law.

         (b) COMPLIANCE WITH SECURITIES LAWS. Each Option and Award shall be
subject to the requirement that if, at any time, counsel to the Company shall
determine that the listing, registration or qualification of the shares subject
to such Option or Award upon any securities exchange or under any state or
federal law, or the consent or approval of any governmental or regulatory body,
or that the disclosure of non-public information or the satisfaction of any
other condition is necessary as a condition of, or in connection with, the
issuance or purchase of shares thereunder, such Option may not be exercised, in
whole or in part, nor such Award issued, unless such listing, registration,
qualification, consent or approval, or satisfaction of such condition have been
effected or obtained on conditions acceptable to the Board of Directors. Nothing
herein shall be deemed to require the Company to apply for or to obtain such
listing, registration or qualification, or to satisfy such condition.

16.      GOVERNING LAW

         The Plan and all determinations made and actions taken pursuant hereto
shall be governed by the laws of the Commonwealth of Massachusetts.



                                            Adopted by the Board of Directors on
                                            June 27, 1994 

                                            Adopted by the Stockholders on
                                            August 12, 1994.



                                       -7-

<PAGE>   9



                                    Exhibit A
                                    ---------

                               BIOTRANSPLANT, INC.

                      NON-STATUTORY STOCK OPTION AGREEMENT
                      ------------------------------------

         1.       GRANTS OF OPTIONS. BioTransplant, Inc. a Delaware corporation 
(the "Company"), hereby grants to ____________________ (the "Optionee"), an
option, pursuant to the Company's 1994 Directors' Equity Plan (the "Plan"), to
purchase an aggregate of 12,500 shares of Common Stock, $.01 par value per share
("Common Stock"), of the Company at a price of $_______ per share, purchasable
as set forth in and subject to the terms and conditions of this option and the
Plan. Except where the context otherwise requires, the term "Company" shall
include the parent and all present and future subsidiaries of the Company as
defined in Sections 424(e) and 424(f) of the Internal Revenue Code of 1986, as
amended or replaced from time to time (the "Code").

         2.       NON-STATUTORY STOCK OPTION. This option is not intended to 
qualify as an incentive stock option within the meaning of Section 422 of the
Code.

         3.       EXERCISE OF OPTION AND PROVISIONS FOR TERMINATION.

                  (a) VESTING SCHEDULE. Except as otherwise provided in this
Agreement, this option may be exercised prior to the tenth anniversary of the
date of grant (hereinafter the "Expiration Date") in installments as to not more
than the number of shares set forth in the table below during the respective
installment periods set forth in the table below (less any shares for which this
option was exercised in any prior installment periods).



<TABLE>
<CAPTION>
                                             Number of Shares as to which
           Exercise Prior                        Option is Exercisable
------------------------------------         ----------------------------
<S>                                                     <C>
Prior to _________________, 19__                           -0-
On or after ______________, 19__ but
prior to _________________, 19__                         2,500
On or after ______________, 19__ but
prior to _________________, 19__                         5,000
On or after ______________, 19__ but
prior to _________________, 19__                         7,500
On or after ______________, 19__ but
prior to _________________, 19__                        10,000
On or after ______________, 19__ but                    12,500
prior to _________________, 19__
</TABLE>



                                      -8-
<PAGE>   10


The right of exercise shall be cumulative so that if the option is not exercised
to the maximum extent permissible during any exercise period, it shall be
exercisable, in whole or in part, with respect to all shares not so purchased at
any time prior to the Expiration Date or the earlier termination of this option.
This option may not be exercised at any time on or after the Expiration Date.

                  (b) EXERCISE PROCEDURE. Subject to the conditions set forth in
this Agreement, this option shall be exercised by the Optionee's delivery of
written notice of exercise to the Treasurer of the Company, specifying the
number of shares to be purchased and the purchase price to be paid therefor and
accompanied by payment in full in accordance with Section 4. Such exercise shall
be effective upon receipt by the Treasurer of the Company of such written notice
together with the required payment. The Optionee may purchase less than the
number of shares covered hereby, provided that no partial exercise of this
option may be for any fractional share or for fewer than ten whole shares.

                  (c) CONTINUOUS RELATIONSHIP WITH THE COMPANY REQUIRED. Except
as otherwise provided in this Section 3, this option may not be exercised unless
the Optionee, at the time he or she exercises this option, is, and has been at
all times since the date of grant of this option, a director of the Company (an
"Eligible Optionee").

                  (d) EXERCISE PERIOD UPON TERMINATION OF RELATIONSHIP WITH THE
COMPANY. If the Optionee ceases to be an Eligible Optionee for any reason, then,
except as provided in paragraph (e) below, the right to exercise this option
shall terminate three months after such cessation.

                  (e) EXERCISE PERIOD UPON DEATH OR DISABILITY. If the Optionee
dies or becomes disabled (within the meaning of Section 22(e)(3) of the Code)
prior to the Expiration Date while he or she is an Eligible Optionee of the
Company, or if the Optionee dies within three months after the Optionee ceases
to be an Eligible Optionee, this option shall be exercisable, within the period
of one year following the date or death or disability of the Optionee (but in no
event after the Expiration Date), by the Optionee or by the person to whom this
option is transferred by will or the laws of descent and distribution, PROVIDED
THAT this option shall be exercisable only to the extent that this option was
exercisable by the Optionee on the date of his or her death or disability.
Except as otherwise indicated by the context, the term "Optionee", as used in
this option, shall be deemed to include the estate of the Optionee or any person
who acquires the right to exercise this option by bequest or inheritance or
otherwise by reason of the death of the Optionee.




                                       -9-

<PAGE>   11



         4.       PAYMENT OF PURCHASE PRICE.

                  (a) METHOD OF PAYMENT. Payment of the purchase price for
shares purchased upon exercise of this option shall be made (i) by delivery to
the Company of cash or a check to the order of the Company in an amount equal to
the purchase price of such shares, (ii) by delivery to the Company of shares of
Common Stock of the Company then owned by the Optionee having a fair market
value equal in amount to the purchase price of such shares, or (iii) by any
combination of such methods of payment.

                  (b) VALUATION OF SHARES OR OTHER NON-CASH CONSIDERATION
TENDERED IN PAYMENT OF PURCHASE PRICE. For the purposes hereof, the fair market
value of any share of the Company's Common Stock or other non-cash consideration
which may be delivered to the Company in exercise of this option shall be
determined in such manner as may be prescribed by the Board of Directors of the
Company.

                  (c) DELIVERY OF SHARES TENDERED IN PAYMENT OF PURCHASE PRICE.
If the Optionee exercises options by delivery of shares of Common Stock of the
Company, the certificate or certificates representing the shares of such Common
Stock of the Company to be delivered shall be duly executed in blank by the
Optionee or shall be accompanied by a stock power duly executed in blank
suitable for purposes of transferring such shares to the Company. Fractional
shares of such Common Stock of the Company will not be accepted in payment of
the purchase price of shares acquired upon exercise of this option.

                  (d) RESTRICTIONS ON USE OF OPTION STOCK. Notwithstanding the
foregoing, no shares of Common Stock of the Company may be termed in payment of
the purchase price of shares purchased upon exercise of this option if the
shares to be so tendered were acquired within twelve (12) months before the date
of such tender, through the exercise of an option granted under the Plan or any
other stock option or restricted stock plan of the Company.

         5.       DELIVERY OF SHARES; COMPLIANCE WITH SECURITY LAWS, ETC.

                  (a) GENERAL. The Company shall, upon payment of the option
price for the number of shares purchased and paid for, make prompt delivery of
such shares to the Optionee, PROVIDED THAT if any law or regulation requires the
Company to take any action with respect to such shares before the issuance
thereof, then the date of delivery of such shares shall be extended for the
period necessary to complete such action.

                  (b) LISTING, QUALIFICATIONS, ETC. This option shall be subject
to the requirement that if, at any time, counsel to the Company shall determine
that the listing, registration or qualification of the shares subject hereto
upon any securities

                                      -10-

<PAGE>   12



exchange or under any state or federal law, or the consent or approval of any
governmental or regulatory body, or that the disclosure of non-public
information or the satisfaction of any other condition is necessary as a
condition of, or in the connection with, the issuance or purchase of shares
hereunder, this option may not be exercised, in whole or in part, unless such
listing, registration, qualification, consent or approval, disclosure or
satisfaction of such other condition shall have been effected or obtained on
terms acceptable to the Board of Directors. Nothing herein shall be deemed to
require the Company to apply for, effect or obtain such listing, registration,
qualification, or disclosure, or to satisfy such other condition.

         6.       NONTRANSFERABILITY OF OPTION. Except as provided in 
paragraph (e) of Section 3, this option is personal and no rights granted
hereunder may be transferred, assigned, pledged or hypothecated in any way
(whether by operation of law or otherwise) nor shall any such rights be subject
to execution, attachment or similar process. Upon any attempt to transfer,
assign, pledge, hypothecate or otherwise dispose of this option or of such
rights contrary to the provisions hereof, or upon the levy of any attachment or
similar process upon this option or such rights, this option and such rights
shall, at the election of the Company, become null and void.

         7.       NO SPECIAL RETENTION, OR SIMILAR RIGHTS. Nothing contained in
the Plan or this option shall be construed or deemed by any person under any
circumstances to bind the Company to continue the retention of the Optionee as a
director or maintain any other relationship of the Optionee with the Company for
the period within which this option may be exercised.

         8.       RIGHTS AS A SHAREHOLDER. The Optionee shall have no rights as
a shareholder with respect to any shares which may be purchased by exercise of
this option (including, without limitation, any rights to receive dividends or
non-cash distributions with respect to such shares) unless and until a
certificate representing such shares is duly issued and delivered to the
Optionee. No adjustment shall be made for dividends or other rights for which
the record date is prior to the date such stock certificate is issued.

         9.       ADJUSTMENT PROVISIONS.

                  (a) GENERAL. If, through, or as a result of, any merger,
consolidation, sale of all or substantially all of the assets of the Company,
reorganization, recapitalization, reclassification, stock dividend, stock split,
reverse stock split or other similar transaction, (i) the outstanding shares of
Common Stock are increased or decreased or are exchanged for a different number
or kind of shares or other securities of the Company, or (ii) additional shares
or new or different shares or other securities of the Company or other non-cash
assets are distributed with respect to such shares of Common Stock or other
securities, the Optionee shall, with respect to

                                      -11-

<PAGE>   13



this option or any unexercised portion hereof, be entitled to the rights and
benefits, and be subject to the limitations, set forth in Section 10(a) of the
Plan.

                  (b)      LIMITS ON ADJUSTMENTS. No adjustment shall be made
under this Section 9 which would, within the meaning of any applicable provision
of the Code, constitute a modification, extension or renewal of this option or a
grant of additional benefits to the Optionee.

         10.      MERGERS, CONSOLIDATIONS, DISTRIBUTIONS, LIQUIDATIONS, ETC. In
the event of a consolidation or merger or sale of all or substantially all of
the assets of the Company in which outstanding shares of Common Stock are
exchanged for securities cash or other property of any other corporation or
business entity, or in the event of a liquidation of the Company, prior to the
Expiration Date or termination of this option, the Optionee shall, with respect
to this option or any unexercised portion hereof, be entitled to the rights and
benefits, and be subject to the limitations, set forth in Section 10(b) of the
Plan.

         11.      WITHHOLDING TAXES. The Company's obligations to deliver shares
upon the exercise of this option shall be subject to the Optionee's satisfaction
of all applicable federal, state and local income and employment tax withholding
requirements.

         12.      INVESTMENT REPRESENTATIONS; LEGENDS.

                  (a)      REPRESENTATIONS. The Optionee represents, warrants
and covenants that:

                           (i)      Any shares purchased upon exercise of this
                  option shall be acquired for the Optionee's account for
                  investment only, and not with a view to, or for sale in
                  connection with, any distribution of the shares in violation
                  of the Securities Act of 1933 (the "Securities Act"), or any
                  rule or regulation under the Securities Act.

                           (ii)     The Optionee has had such opportunity as he
                  or she has deemed adequate to obtain from representatives of
                  the Company such information as is necessary to permit the
                  Optionee to evaluate the merits and risks of his or her
                  investment in the Company.

                           (iii)    The Optionee is able to bear the economic
                  risk of holding such shares acquired pursuant to the exercise
                  of this option for an indefinite period.

                           (iv)     The Optionee understands that (A) the shares
                  acquired pursuant to the exercise of this option will not be
                  registered under the

                                      -12-

<PAGE>   14



                  Securities Act and are "restricted securities" within the
                  meaning of Rule 144 under the Securities Act; (B) such shares
                  cannot be sold, transferred or otherwise disposed of unless
                  they are subsequently registered under the Securities Act or
                  an exemption from registration is then available; (C) in any
                  event, an exemption from registration under Rule 144 or
                  otherwise under the Securities Act may not be available for at
                  least two years and even then will not be available unless a
                  public market then exists for the Common Stock, adequate
                  information concerning the Company is then available to the
                  public, and other terms and conditions of Rule 144 are
                  complied with; and (D) there is now no registration statement
                  on file with the Securities and Exchange Commission with
                  respect to any stock of the Company and Company has no
                  obligation or current intention to register any shares
                  acquired pursuant to the exercise of this option under the
                  Securities Act.

                           (v) The Optionee agrees that, if the Company offers
                  any of its Common Stock for sale pursuant to registration
                  statement under the Securities Act, the Optionee will not,
                  without the prior written consent of the Company, offer, sell,
                  contract to sell or otherwise dispose of, directly or
                  indirectly, any shares purchased upon exercise of this option
                  for a period of 90 days after the effective date of such
                  registration statement.

By making payment upon exercise of this option, the Optionee shall be deemed to
have reaffirmed, as of the date of such payment, the representations made in
this Section 12.

                  (b)      LEGENDS ON STOCK CERTIFICATE. All stock certificates
representing shares of Common Stock issued to the Optionee upon exercise of this
option shall have affixed thereto legends substantially in the following forms,
in addition to any other legends required by applicable state law:

                  "The shares of stock represented by this certificate have not
                  been registered under the Securities Act of 1933 and may not
                  be transferred, sold or otherwise disposed of in the absence
                  of an effective registration statement with respect to the
                  shares evidenced by this certificate, filed and made effective
                  under the Securities Act of 1933, or an opinion of counsel
                  satisfactory to the Company to the effect that registration
                  under such Act is not required."

                  "The shares of stock represented by this certificate are
                  subject to certain restrictions on transfer contained in an

                                      -13-

<PAGE>   15



                  Option Agreement, a copy of which will be furnished upon
                  request by the issuer."

         13.      MISCELLANEOUS.

                  (a) Except as provided herein, this option may not be amended
or otherwise modified unless evidenced in writing and signed by the Company and
the Optionee.

                  (b) All notices under this option be mailed or delivered by
hand to the parties at their respective addresses set forth beneath their names
below or at such other address as may be designated in writing by either of the
parties to one another.

                  (c) This option shall be governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts.


Date of Grant:                                BIOTRANSPLANT, INC.
____________________, 199__

                                              By:_______________________________

                                              Title:____________________________

                                              Address:  Building 96
                                                        13th Street
                                                        Charlestown Navy Yard
                                                        Charlestown, MA 02129


                                      -14-

<PAGE>   16



                              OPTIONEE'S ACCEPTANCE


         The undersigned hereby accepts the following option and agrees to the
terms and conditions thereof. The undersigned hereby acknowledges receipt of a
copy of the Company's 1994 Directors' Equity Plan.


                                              OPTIONEE


                                              ----------------------------------
                                              Address:



                                      -15-

<PAGE>   17



                                    EXHIBIT B
                                    ---------


                               BIOTRANSPLANT, INC.
                       RESTRICTED STOCK PURCHASE AGREEMENT
                       -----------------------------------

         AGREEMENT made this _____ day of ________, 199_, between BIOTRANSPLANT,
INC., a Delaware corporation (the "Company") and ____________________ (the
"Purchaser").

                                   Background
                                   ----------

         Pursuant to the Company's 1994 Directors' Equity Plan (the "Plan") each
eligible director receives a restricted stock award of 2,500 shares of Common
Stock of the Company upon his or her initial election to the Board of Directors
or the adoption of the Plan, whichever is later. This Agreement is entered into
pursuant to, and is subject to the terms and conditions of, the Plan.

         For valuable consideration, receipt of which is acknowledged, the
parties hereto agree as follows:

         1.       PURCHASE OF SHARES. The Company shall issue and sell to the
Purchaser, and the Purchaser shall purchase from the Company, subject to the
terms and conditions set forth in this Agreement, 2,500 shares (the "Shares") of
common stock, $.01 par value, of the Company ("Common Stock"), at a purchase
price of $.01 per share. The aggregate purchase price for the Shares shall be
paid by the Purchaser by check payable to the order of the Company or such other
method as may be acceptable to the Company. Upon receipt of payment by the
Company for the Shares, the Company shall issue to the Purchaser one or more
certificates in the name of the Purchaser for that number of Shares purchased by
the Purchaser. The Purchaser agrees that the Shares shall be subject to the
Purchase Option set forth in Section 2 of this Agreement and the restrictions on
transfer set forth in Section 4 of this Agreement.

         2.       PURCHASE OPTION.

                  (a) In the event that the Purchaser ceases to serve as a
director of the Company for any reason or no reason, with or without cause,
prior to ______________, 199_, the Company shall have the right and option (the
"Purchase Option") to purchase form the Purchaser, for a sum of $.01 per share
(the "Option Price"), up to that percentage of the Shares as is set forth in the
second column of the table set forth below opposite the period in which the
Purchaser ceases to so serve.


                                      -16-

<PAGE>   18




<TABLE>
<CAPTION>
                                                         Percentage of Shares
If Cessation of Service Occurs:                       Subject to Purchase Option
-------------------------------                       --------------------------
<S>                                                               <C> 
Before ___________________                                        100%

On or after ______________
   but before ____________                                        75%

On or after ______________
   but before ____________                                        50%

On or after ______________
   but before ____________                                        25%

On or after ______________                                        -0-
</TABLE>

         3.       EXERCISE OF PURCHASE OPTION AND CLOSING.

                  (a) The Company may exercise the Purchase Option by delivering
or mailing to the Purchaser (or his estate), in accordance with section 14,
within 60 days after the termination of service as a director of the Purchaser
with the Company, a written notice of exercise of the Purchase Option. Such
notice shall specify the number of Shares to be purchased. If and to the extent
the purchase Option is not so exercised by the giving of such a notice within
such 60-day period, the Purchase Option shall automatically expire and terminate
effective upon the expiration of such 60-day period.

                  (b) Within 10 days after his receipt of the Company's notice
of the exercise of the Purchase Option pursuant to subsection (a) above, the
Purchaser (or his estate) shall tender to the Company at its principal offices
the certificate or certificates representing the Shares which the Company has
elected to purchase, duly endorsed in blank by the Purchaser or with duly
endorsed stock powers attached thereto, all in form suitable for the transfer of
such Shares to the Company. Upon its receipt of such certificate or
certificates, the Company shall deliver or mail to the Purchaser a check in the
amount of the aggregate Option Price therefor.

                  (c) After the time at which any Shares are required to be
delivered to the Company for transfer to the Company pursuant to subsection (b)
above, the Company shall not pay any dividend to the Purchaser on account of
such Shares or permit the Purchaser to exercise any of the privileges or rights
of a stockholder with respect to such Shares, but shall, in so far as permitted
by law, treat the Company as the owner of such Shares.


                                      -17-

<PAGE>   19



                  (d) The Option Price may be payable, at the option of the
Company, in cancellation of all or a portion of any outstanding indebtedness of
the Purchaser to the Company or in cash (by check) or both.

                  (e) The Company shall not purchase any fraction of a Share
upon exercise of a Purchase Option, and any fraction of a Share resulting from a
computation made pursuant to Section 2 of this Agreement shall be rounded to the
nearest whole Share (with any one-half Share being rounded upward).

                  (f) The Company shall have the right to assign or transfer all
or part of its Purchase Option to one or more third parties, including other
stockholders of the Company.

         4.       RESTRICTIONS ON TRANSFERS.

                  (a) Except as otherwise provided in subsection (b) below, the
Purchaser shall not, during the term of the Purchase Option, sell, assign,
transfer, pledge, hypothecate or otherwise dispose of, by operation of law or
otherwise (collectively "transfer"), any of the Shares, or any interest therein,
unless and until such Shares are no longer subject to the Purchase Option.

                  (b) Notwithstanding the foregoing, the Purchaser may transfer
Shares to or for the benefit of any spouse, child or grandchild, or to a trust
for their benefit, PROVIDED that such Shares shall remain subject to this
Agreement (including without limitation the restrictions on transfer set forth
in this Section 4 and the Purchase Option) and such permitted transferee shall,
as a condition to such transfer, deliver to the Company a written instrument
confirming that such transferee shall be bound by all of the terms and
conditions of this Agreement.

         5.       EFFECT OF PROHIBITED TRANSFER. The Company shall not be
required (a) to transfer on its books any of the Shares; which shall have been
sold or transferred in violation of any of the provisions set forth in this
Agreement, or (b) to treat as owner of such Shares or to pay dividends to any
transferee to whom any such Shares shall have been sold or transferred.

         6.       RESTRICTIVE LEGEND. All certificates representing Shares shall
have affixed thereto a legend in substantially the following form, in addition
to any other legends that may be required under federal or state securities
laws:

                  "The shares of stock represented by this certificate are
                  subject to restrictions on transfer and an option to purchase
                  set forth in a certain Restricted Stock Purchase Agreement
                  between the corporation and the registered owner of these
                  share (or his predecessor in interest), and such Agreement


                                      -18-

<PAGE>   20



                  is available for inspection without charge at the office of
                  the Secretary of the corporation."

         7.       INVESTMENT REPRESENTATIONS. The Purchaser represents, warrants
and covenants as follows:

                  (a) The Purchaser is purchasing the Shares for his own account
for investment only, and not with a view to, or for sale in connection with, any
distribution of the Shares in violation of the Securities Act of 1933 (the
"Securities Act"), or any rule or regulation under the Securities Act.

                  (b) The Purchaser has had such opportunity as he has deemed
adequate to obtain from representatives of the Company such information as is
necessary to permit him to evaluate the merits and risks of his investment in
the Company.

                  (c) The Purchaser has sufficient experience in business,
financial and investment matters to be able to evaluate the risks involved in
the purchase of the Shares and to make an informed investment decision with
respect to such purchase.

                  (d) The Purchaser can afford a complete loss of the value of
the Shares and is able to bear the economic risk of holding such Shares for an
indefinite period.

                  (e) The Purchaser understands that (i) the Shares have not
been registered under the Securities Act and are "restricted securities" within
the meaning of Rule 144 under the Securities Act, (ii) the Shares cannot be
sold, transferred or otherwise disposed of unless they are subsequently
registered under the Securities Act or an exemption from registration is then
available; (iii) in any event, the exemption from registration under Rule 144
will not be available for at least two years and even then will not be available
unless a public market then exists for the Common Stock, adequate information
concerning the Company is then available to the public, and other terms and
conditions of Rule 144 are complied with; and (iv) there is now no registration
statement on file with the Securities and Exchange Commission with respect to
any stock of the Company and the Company has no obligations or current intention
to register the Shares under the Securities Act.

                  (f) A legend substantially in the following form will be
placed on the certificate representing the Shares:

                  "The shares represented by this certificate have not been
                  registered under the Securities Act of 1933, as amended, and
                  may not be sold, transferred or otherwise disposed of in the
                  absence of an effective registration statement under



                                      -19-

<PAGE>   21



                  such Act or an opinion of counsel satisfactory to the
                  corporation to the effect that such
                  registration is not required."

         8.       ADJUSTMENTS FOR STOCK SPLITS, STOCK DIVIDENDS, ETC.

                  (a) If from time to time during the term of the Purchase
Option there is any stock split-up, stock dividend, stock distribution or other
reclassification of the Common Stock of the Company, any and all new,
substituted or additional securities to which the Purchaser is entitled by
reason of his ownership of the Shares shall be immediately subject to the
Purchase Option, the restrictions on transfer and other provisions of this
Agreement in the same manner and to the same extent as the Shares, and the
Option Price shall be appropriately adjusted.

                  (b) If the Shares are converted into or exchanged for, or
stockholders of the Company receive by reason of any distribution in total or
partial liquidation, securities of another corporation, or other property
(including cash), pursuant to any merger of the Company or acquisition of its
assets, then the rights of the Company under this Agreement shall inure to the
benefit of the Company's successor and this Agreement shall apply to the
securities or other property received upon such conversion, exchange or
distribution in the same manner and to the same extent as the Shares.

         9.       WITHHOLDING TAXES.

                  (a) The Purchaser acknowledges and agrees that the Company has
the right to deduct from payments of any kind otherwise due to the Purchaser any
federal, state or local taxes of any kind required by law to be withheld with
respect to the purchase of the Shares by the Purchaser.

                  (b) If the Purchaser elects, in accordance with Section 83(b)
of the internal Revenue Code of 1986, as amended, to recognize ordinary income
in the year of acquisition of the Shares, the Company will require at the time
of such election an additional payment for withholding tax purposes based on the
difference, if any, between the purchase price for such Shares and their fair
market value of such Shares as of the day immediately preceding the date of the
purchase of such Shares by the Purchaser.

         10.      SEVERABILITY. The invalidity or unenforceability of any 
provision of this Agreement shall not affect the validity or enforceability of
any other provision of this Agreement, and each other provision of this
Agreement shall be severable and enforceable to the extent permitted by law.


                                      -20-

<PAGE>   22



         11. WAIVER. Any provision contained in this Agreement may be waived,
either generally or in any particular instance, by the Board of Directors of the
Company.

         12. BINDING EFFECT. This Agreement shall be binding upon and inure to
the benefit of the Company and the Purchaser and their respective heirs,
executors, administrators, legal representatives, successors and assigns,
subject to the restrictions on transfer set forth in Section 4 of this
Agreement.

         13. NO RIGHTS TO CONTINUED BOARD SERVICE. Nothing contained in this
Agreement shall be construed as giving the Purchaser any right to be retained,
in any position, as a director of the Company.

         14. NOTICE. All notices required or permitted hereunder shall be in
writing and deemed effectively given upon personal delivery or upon deposit in
the United States Post Office, by registered or certified mail, postage prepaid,
addressed to the other party hereto at the address shown beneath his or its
respective signature to this Agreement, or at such other address or addresses as
either party shall designate to the other in accordance with this Section 14.

         15. PRONOUNS. Whenever the context may require, any pronouns used in
this Agreement shall include the corresponding masculine, feminine or neuter
forms, and the singular form of nouns and pronouns shall include the plural, and
vice versa.

         16. ENTIRE AGREEMENT. This Agreement constitutes the entire agreement
between the parties, and supersedes all prior agreements and understandings,
relating to the subject matter of this Agreement.

         17. AMENDMENT. This Agreement may be amended or modified only by a
written instrument executed by both the Company and the Purchaser.

         18. GOVERNING LAW. This Agreement shall be construed, interpreted and
enforced in accordance with the laws of the Commonwealth of Massachusetts.

         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year first above written.


                                      -21-

<PAGE>   23





                                              BIOTRANSPLANT, INC.

                                              By:
                                                  ------------------------------

                                              Address: Building 96
                                                       13th Street
                                                       Charlestown Navy Yard
                                                       Charlestown, MA 02129


                                              PURCHASER


                                              ----------------------------------
                                              Address:





                                      -22-