Document:

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                                                                 Exhibit 10.22

                           BIOTRANSPLANT INCORPORATED

                            1997 STOCK INCENTIVE PLAN

1.     PURPOSE

       The purpose of this 1997 Stock Incentive Plan (the "Plan") of
BioTransplant Incorporated, a Delaware corporation (the "Company"), is to
advance the interests of the Company's stockholders by enhancing the Company's
ability to attract, retain and motivate persons who make (or are expected to
make) important contributions to the Company by providing such persons with
equity ownership opportunities and performance-based incentives and thereby
better aligning the interests of such persons with those of the Company's
stockholders. Except where the context otherwise requires, the term "Company"
shall include any present or future subsidiary corporations of BioTransplant
Incorporated as defined in Section 424(f) of the Internal Revenue Code of 1986,
as amended, and any regulations promulgated thereunder (the "Code").

2.     ELIGIBILITY

       All of the Company's employees, officers, directors, consultants and
advisors are eligible to be granted options, restricted stock, or other
stock-based awards (each, an "Award") under the Plan. Any person who has been
granted an Award under the Plan shall be deemed a "Participant".

3.     ADMINISTRATION, DELEGATION

       (a) ADMINISTRATION BY BOARD OF DIRECTORS. The Plan will be administered
by the Board of Directors of the Company (the "Board"). The Board shall have
authority to grant Awards and to adopt, amend and repeal such administrative
rules, guidelines and practices relating to the Plan as it shall deem advisable.
The Board may correct any defect, supply any omission or reconcile any
inconsistency in the Plan or any Award in the manner and to the extent it shall
deem expedient to carry the Plan into effect and it shall be the sole and final
judge of such expediency. All decisions by the Board shall be made in the
Board's sole discretion and shall be final and binding on all persons having or
claiming any interest in the Plan or in any Award. No director or person acting
pursuant to the authority delegated by the Board shall be liable for any action
or determination relating to or under the Plan made in good faith.

       (b) DELEGATION TO EXECUTIVE OFFICERS. To the extent permitted by
applicable law, the Board may delegate to one or more executive officers of the
Company the power to make Awards and exercise such other powers under the Plan
as the Board may determine, provided that the Board shall fix the maximum number
of shares subject to Awards and the maximum number of shares for any one
Participant to be made by such executive officers.

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       (c) APPOINTMENT OF COMMITTEES. To the extent permitted by applicable law,
the Board may delegate any or all of its powers under the Plan to one or more
committees or subcommittees of the Board (a "Committee"). If and when the common
stock, $0.01 par value per share, of the Company (the "Common Stock") is
registered under the Securities Exchange Act of 1934 (the "Exchange Act"), the
Board shall appoint one such Committee of not less than two members, each member
of which shall be an "outside director" within the meaning of Section 162(m) of
the Code and a "non-employee director" as defined in Rule 16b-3 promulgated
under the Exchange Act." All references in the Plan to the "Board" shall mean
the Board or a Committee of the Board or the executive officer referred to in
Section 3(b) to the extent that the Board's powers or authority under the Plan
have been delegated to such Committee or executive officer.

4.     STOCK AVAILABLE FOR AWARDS

       (a) NUMBER OF SHARES. Subject to adjustment under Section 4(c), Awards
may be made under the Plan for up to 750,000 shares of Common Stock. If any
Award expires or is terminated, surrendered or canceled without having been
fully exercised or is forfeited in whole or in part or results in any Common
Stock not being issued, the unused Common Stock covered by such Award shall
again be available for the grant of Awards under the Plan, subject, however, in
the case of Incentive Stock Options (as hereinafter defined), to any limitation
required under the Code. Shares issued under the Plan may consist in whole or in
part of authorized but unissued shares or treasury shares.

       (b) PER-PARTICIPANT LIMIT. Subject to adjustment under Section 4(c), for
Awards granted after the Common Stock is registered under the Exchange Act, the
maximum number of shares with respect to which an Award may be granted to any
Participant under the Plan shall be 100,000 per calendar year. The
Per-Participant limit described in this Section 4(b) shall be construed and
applied consistently with Section 162(m) of the Code.

       (c) ADJUSTMENT TO COMMON STOCK. In the event of any stock split, stock
dividend, recapitalization, reorganization, merger, consolidation, combination,
exchange of shares, liquidation, spin-off or other similar change in
capitalization or event, or any distribution to holders of Common Stock other
than a normal cash dividend, (i) the number and class of securities available
under this Plan, (ii) the number and class of security and exercise price per
share subject to each outstanding Option, (iii) the repurchase price per
security subject to each outstanding Restricted Stock Award, and (iv) the terms
of each other outstanding stock-based Award shall be appropriately adjusted by
the Company (or substituted Awards may be made, if applicable) to the extent the
Board shall determine, in good faith, that such an adjustment (or substitution)
is necessary and appropriate. If this Section 4(c) applies and Section 8(e)(1)
also applies to any event, Section 8(e)(1) shall be applicable to such event,
and this Section 4(c) shall not be applicable.

5.     STOCK OPTIONS

       (a) GENERAL. The Board may grant options to purchase Common Stock (each,
an "Option") and determine the number of shares of Common Stock to be covered by
each Option,

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the exercise price of each Option and the conditions and limitations applicable
to the exercise of each Option, including conditions relating to applicable
federal or state securities laws, as it considers necessary or advisable. An
Option which is not intended to be an Incentive Stock Option (as hereinafter
defined) shall be designated a "Nonstatutory Stock Option".

       (b) INCENTIVE STOCK OPTIONS. An Option that the Board intends to be an
"incentive stock option" as defined in Section 422 of the Code (an "Incentive
Stock Option") shall only be granted to employees of the Company and shall be
subject to and shall be construed consistently with the requirements of Section
422 of the Code. The Company shall have no liability to a Participant, or any
other party, if an Option (or any part thereof) which is intended to be an
Incentive Stock Option is not an Incentive Stock Option.

       (c) EXERCISE PRICE. The Board shall establish the exercise price at the
time each Option is granted and specify it in the applicable option agreement.

       (d) DURATION OF OPTIONS. Each Option shall be exercisable at such times
and subject to such terms and conditions as the Board may specify in the
applicable option agreement.

       (e) EXERCISE OF OPTION. Options may be exercised only by delivery to the
Company of a written notice of exercise signed by the proper person together
with payment in full as specified in Section 5(f) for the number of shares for
which the Option is exercised.

       (f) PAYMENT UPON EXERCISE. Common Stock purchased upon the exercise of an
Option granted under the Plan shall be paid for as follows:

            (1)  in cash or by check, payable to the order of the Company;

            (2)  except as the Board may otherwise provide in an Option
                 Agreement, delivery of an irrevocable and unconditional
                 undertaking by a credit worthy broker to deliver promptly to
                 the Company sufficient funds to pay the exercise price, or
                 delivery by the Participant to the Company of a copy of
                 irrevocable and unconditional instructions to a credit worthy
                 broker to deliver promptly to the Company cash or a check
                 sufficient to pay the exercise price;

            (3)  to the extent permitted by the Board and explicitly provided in
                 an Option Agreement (i) by delivery of shares of Common Stock
                 owned by the Participant valued at their fair market value as
                 determined by the Board in good faith ("Fair Market Value"),
                 which Common Stock was owned by the Participant at least six
                 months prior to such delivery, (ii) by delivery of a promissory
                 note of the Participant to the Company on terms determined by
                 the Board, or (iii) by payment of such other lawful
                 consideration as the Board may determine; or

            (4)  any combination of the above permitted forms of payment.

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6.     RESTRICTED STOCK

       (a) GRANTS. The Board may grant Awards entitling recipients to acquire
shares of Common Stock, subject to the right of the Company to repurchase all or
part of such shares at their issue price or other stated or formula price (or to
require forfeiture of such shares if issued at no cost) from the recipient in
the event that conditions specified by the Board in the applicable Award are not
satisfied prior to the end of the applicable restriction period or periods
established by the Board for such Award (each, "Restricted Stock Award").

       (b) TERMS AND CONDITIONS. The Board shall determine the terms and
conditions of any such Restricted Stock Award, including the conditions for
repurchase (or forfeiture) and the issue price, if any. Any stock certificates
issued in respect of a Restricted Stock Award shall be registered in the name of
the Participant and, unless otherwise determined by the Board, deposited by the
Participant, together with a stock power endorsed in blank, with the Company (or
its designee). At the expiration of the applicable restriction periods, the
Company (or such designee) shall deliver the certificates no longer subject to
such restrictions to the Participant or if the Participant has died, to the
beneficiary designated, in a manner determined by the Board, by a Participant to
receive amounts due or exercise rights of the Participant in the event of the
Participant's death (the "Designated Beneficiary"). In the absence of an
effective designation by a Participant, Designated Beneficiary shall mean the
Participant's estate.

7.     OTHER STOCK-BASED AWARDS

       The Board shall have the right to grant other Awards based upon the
Common Stock having such terms and conditions as the Board may determine,
including the grant of shares based upon certain conditions, the grant of
securities convertible into Common Stock and the grant of stock appreciation
rights.

8.     GENERAL PROVISIONS APPLICABLE TO AWARDS

       (a) TRANSFERABILITY OF AWARDS. Except as the Board may otherwise
determine or provide in an Award, Awards shall not be sold, assigned,
transferred, pledged or otherwise encumbered by the person to whom they are
granted, either voluntarily or by operation of law, except by will or the laws
of descent and distribution, and, during the life of the Participant, shall be
exercisable only by the Participant. References to a Participant, to the extent
relevant in the context, shall include references to authorized transferees.

       (b) DOCUMENTATION. Each Award under the Plan shall be evidenced by a
written instrument in such form as the Board shall determine. Each Award may
contain terms and conditions in addition to those set forth in the Plan.

       (c) BOARD DISCRETION. Except as otherwise provided by the Plan, each type
of Award may be made alone or in addition or in relation to any other type of
Award. The terms of each type of Award need not be identical, and the Board need
not treat Participants uniformly.

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       (d) TERMINATION OF STATUS. The Board shall determine the effect on an
Award of the disability, death, retirement, authorized leave of absence or other
change in the employment or other status of a Participant and the extent to
which, and the period during which, the Participant, the Participant's legal
representative, conservator, guardian or Designated Beneficiary may exercise
rights under the Award.

       (e) ACQUISITION EVENTS

            (1) CONSEQUENCES OF ACQUISITION EVENTS. Except to the extent
otherwise provided in the instrument evidencing the Award or in any other
agreement between the Participant and the Company, upon the occurrence of an
Acquisition Event or with respect to Options or any other similar Awards only,
upon the execution by the Company of any agreement with respect to an
Acquisition Event, (i) the Board shall provide written notice to the
Participants that all Options then outstanding shall become immediately
exercisable in full as of a specified date (the "Acceleration Date") prior to
the Acquisition Event and will terminate immediately prior to the consummation
of such Acquisition Event, except to the extent exercised by the Participants
between the Acceleration Date and the consummation of such Acquisition Event;
(ii) all Restricted Stock then outstanding shall become immediately free of all
restrictions; (iii) all other stock-based Awards all become immediately
exercisable, realizable or vested in full, or shall be immediately free of all
restrictions or conditions, as the case may be.

            (2) An "Acquisition Event" shall mean (a) any merger or
consolidation which results in the voting securities of the Company outstanding
immediately prior thereto continuing to represent (either by remaining
outstanding or by being converted into voting securities of the surviving or
acquiring entity) less than 50% of the combined voting power of the voting
securities of the Company or such surviving or acquiring entity outstanding
immediately after such merger or consolidation; (b) any sale of all or
substantially all of the assets of the Company; (c) the complete liquidation of
the Company; or (d) the acquisition of "beneficial ownership" (as defined in
Rule 13-d-3 under the Exchange Act) of securities of the Company representing
50% or more of the combined voting power of the Company's then outstanding
securities (other than through a merger or consolidation or an acquisition of
securities directly from the Company) by any "person", as such term is used in
Sections 13(d) and 14(d) of the Exchange Act other than the Company, any trustee
or other fiduciary holding securities under an employee benefit plan of the
Company or any corporation owned directly or indirectly by the stockholders of
the Company in substantially the same proportion as their ownership of the stock
of the Company.

            (3) ASSUMPTION OF OPTIONS UPON CERTAIN EVENTS. The Board may grant
Awards under the Plan in substitution for stock and stock-based awards held by
employees of another corporation who become employees of the Company as a result
of a merger or consolidation of the employing corporation with the Company or
the acquisition by the Company of property or stock of the employing
corporation. The substitute Awards shall be granted on such terms and conditions
as the Board considers appropriate in the circumstances.

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       (f) WITHHOLDING. Each Participant shall pay to the Company, or make
provision satisfactory to the Board for payment of, any taxes required by law to
be withheld in connection with Awards to such Participant no later than the date
of the event creating the tax liability. The Board may allow Participants to
satisfy such tax obligations in whole or in part in shares of Common Stock,
including shares retained from the Award creating the tax obligation, valued at
their Fair Market Value. The Company may, to the extent permitted by law, deduct
any such tax obligations from any payment of any kind otherwise due to a
Participant.

       (g) AMENDMENT OF AWARD. The Board may amend, modify or terminate any
outstanding Award, including but not limited to, substituting therefor another
Award of the same or a different type, changing the date of exercise or
realization, and converting an Incentive Stock Option to a Nonstatutory Stock
Option, provided that the Participant's consent to such action shall be required
unless the Board determines that the action, taking into account any related
action, would not materially and adversely affect the Participant.

       (h) CONDITIONS ON DELIVERY OF STOCK. The Company will not be obligated to
deliver any shares of Common Stock pursuant to the Plan or to remove
restrictions from shares previously delivered under the Plan until (i) all
conditions of the Award have been met or removed to the satisfaction of the
Company, (ii) in the opinion of the Company's counsel, all other legal matters
in connection with the issuance and delivery of such shares have been satisfied,
including any applicable securities laws and any applicable stock exchange or
stock market rules and regulations, and (iii) the Participant has executed and
delivered to the Company such representations or agreements as the Company may
consider appropriate to satisfy the requirements of any applicable laws, rules
or regulations.

       (i) ACCELERATION. The Board may at any time provide that any Options
shall become immediately exercisable in full or in part, that any Restricted
Stock Awards shall be free of all restrictions or that any other stock-based
Awards may become exercisable in full or in part or free of some or all
restrictions or conditions, or otherwise realizable in full or in part, as the
case may be.

9.     MISCELLANEOUS

       (a) NO RIGHT TO EMPLOYMENT OR OTHER STATUS. No person shall have any
claim or right to be granted an Award, and the grant of an Award shall not be
construed as giving a Participant the right to continued employment or any other
relationship with the Company. The Company expressly reserves the right at any
time to dismiss or otherwise terminate its relationship with a Participant free
from any liability or claim under the Plan, except as expressly provided in the
applicable Award.

       (b) NO RIGHTS AS STOCKHOLDER. Subject to the provisions of the applicable
Award, no Participant or Designated Beneficiary shall have any rights as a
stockholder with respect to any shares of Common Stock to be distributed with
respect to an Award until becoming the record holder of such shares.

       (c) EFFECTIVE DATE AND TERM OF PLAN. The Plan shall become effective

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on the date on which it is adopted by the Board, but no Award granted to a
Participant designated as subject to Section 162(m) by the Board shall become
exercisable, vested or realizable, as applicable to such Award, unless and until
the Plan has been approved by the Company's stockholders. No Awards shall be
granted under the Plan after the completion of ten years from the earlier of (i)
the date on which the Plan was adopted by the Board or (ii) the date the Plan
was approved by the Company's stockholders, but Awards previously granted may
extend beyond that date.

       (d) AMENDMENT OF PLAN. The Board may amend, suspend or terminate the Plan
or any portion thereof at any time, provided that no Award granted to a
Participant designated as subject to Section 162(m) by the Board after the date
of such amendment shall become exercisable, realizable or vested, as applicable
to such Award (to the extent that such amendment to the Plan was required to
grant such Award to a particular Participant), unless and until such amendment
shall have been approved by the Company's stockholders.

       (e) STOCKHOLDER APPROVAL. For purposes of this Plan, stockholder approval
shall mean approval by a vote of the stockholders in accordance with the
requirements of Section 162(m) of the Code.

       (f) GOVERNING LAW. The provisions of the Plan and all Awards made
hereunder shall be governed by and interpreted in accordance with the laws of
the State of Delaware, without regard to any applicable conflicts of law.

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                               AMENDMENT NO. 1 TO
                            1997 STOCK INCENTIVE PLAN
                                       OF
                           BIOTRANSPLANT INCORPORATED

       The 1997 Stock Incentive Plan, as amended (the "Plan") of BioTransplant
Incorporated be, and hereby by is, amended as follows:

       1.   Section 4, paragraph (a) is deleted in its entirety and the
            following is substituted in its place:

       "(a) NUMBER OF SHARES. Subject to adjustment under Section 4(c), Awards
may be made under the Plan for up to 1,500,000 shares of Common Stock. If
anyAward expires or is terminated, surrendered or cancelled without having been
fully exercised or is forfeited in whole or in part or results in any Common
Stock not being issued, the unused Common Stock covered by such Award shall
again be available for the grant of Awards under the Plan, subject, however, in
the case of Incentive Stock Options (as hereinafter defined), to any limitation
required under the Code. Shares issued under the Plan may consist in whole or in
part of authorized but unissued shares or treasury shares."

                                       Adopted by the Board of Directors on
                                       February 11, 1999

                                       Adopted by the Stockholders on
                                       May 24, 1999<PAGE>

                                                                   Exhibit 10.30

                      DATED this 20th day of December 1996

                                     BETWEEN

                               BIOTRANSPLANT INC.
                                     ("BTI")

                                      -and-

                            CASTELLA RESEARCH PTY LTD
                                  ("Castella")

                                      -and-

                             SECURE SCIENCES PTY LTD
                                   ("Secure")

                                      -and-

                           STEM CELL SCIENCES PTY LTD
                                 ("the Company")

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                AGREEMENT TO FURTHER VARY SHAREHOLDERS' AGREEMENT

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

                                 HOLDING REDLICH
                             LAWYERS AND CONSULTANTS

350 Williams Street                                     Level 12, Chifley Tower
Melbourne VIC 3000                                      2 Chifley Square
                                                        Sydney NSW 2000

Tel (03) 93219999                                       Tel (02) 234 4444
Fax (03) 93219900                                       Fax (02) 234 4400
Ref: CLSCS109.106

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                                    CONTENTS

<TABLE>
<S>     <C>                                                                                                   <C>

   1.    DEFINITIONS AND INTERPRETATION...........................................................................5

         1.1  Definitions.........................................................................................5

         1.2  Interpretation......................................................................................5

         1.3  Recitals............................................................................................5

   2.    ISSUE OF SHARES..........................................................................................5

         2.1  Issue of Shares to BTI..............................................................................5

         2.2  Issue of Shares to Castella.........................................................................5

         2.3  Issue of Shares to Secure...........................................................................6

   3.    ISSUE OF OPTIONS.........................................................................................6

         3.1  Issue of Options to BTI.............................................................................6

         3.2  Issue of Option to Castella.........................................................................6

         3.3  Issue of Option to Secure...........................................................................6

   4.    VARIATION OF SHAREHOLDERS' AGREEMENT.....................................................................6

         4.1  Variation...........................................................................................6

         4.2  Confirmation of Shareholders' Agreement.............................................................6

         4.3  Acknowledgements....................................................................................7
</TABLE>

                                      -2-

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AGREEMENT TO FURTHER VARY SHAREHOLDERS' AGREEMENT dated the    day
of          , 1996

BETWEEN:

         BIOTRANSPLANT INCORPORATED a corporation organised and existing under
         the laws of the State of Delaware and having its principal office 13
         Pipp Street, Building 96, Navy Yard, Charlestown, MA, United States of
         America

                                                                         ("BTI")

AND:     CASTELLA RESEARCH PTY LTD (ACN 006 125 364) of 587 Whitehorse Road,
         Mont Albert, Victoria, Australia in its own capacity and as trustee of
         the M.R. Brandon Family Trust

                                                                    ("CASTELLA")

AND:     SECURE SCIENCES PTY LTD (ACN 064 139 948) of Level 10, 420 St. Kilda
         Road, Melbourne, Victoria, Australia in its own capacity and as
         trustee of the Secure Sciences Unit Trust

                                                                      ("SECURE")

AND:     STEM CELL SCIENCES PTY LTD (ACN 063 293 130) of Level 10, 420 St.
         Kilda Road, Melbourne, Victoria, Australia

                                                                 ("THE COMPANY")

RECITALS

A.       By a Shareholders' Agreement dated 5 April, 1994 between BTI, Castella,
         Secure and the Company (THE "SHAREHOLDERS' AGREEMENT") the Shareholders
         recorded their agreement as to how the Company would be owned,
         controlled and funded by them.

B.       Pursuant to Clause 2.5 of the Shareholders' Agreement the Company
         issued to BTI the First BTI Option and the Second BTI Option.

C.       The First BTI Option was to be exercised on or before the first
         anniversary of the Effective Date.

D.       Pursuant to Clause 2.5(b) of the Shareholders' Agreement if the First
         BTI Option was not exercised on or before the first anniversary of the
         Effective Date, the Second BTI Option would lapse.

E.       BTI did not exercise the First BTI Option on or before the first
         anniversary of the Effective Date (the anniversary occurring on 5
         April, 1995) and accordingly, the Second BTI Option lapsed.

                                      -3-
<PAGE>

F.       Each of Castella and Secure exercised the options granted to them
         pursuant to Clause 2.5(a)(iii) and (v), respectively, by the first
         anniversary of the Effective Date.

G.       Pursuant to a Variation of the Shareholders' Agreement dated 2
         February, 1996 between BTI, Castella, Secure and the Company ("THE
         FIRST VARIATION"):

         (a)      BTI subscribed for and the Company issued to BU a further 300
                  "A" Shares in the capital of the Company; and

         (b)      the Company issued to BTI an option to subscribe for a further
                  600 "A" Shares in the capital of the Company at a price of
                  $4,685.00 per share at any time on or before 1 July, 1996
                  ("THE NEW OPTION").

H.       Neither Castella or Secure exercised the options granted to them
         pursuant to, respectively, Clause 2.5(a)(iv) and (vi) of the
         Shareholders' Agreement (respectively, "the Second Castella Option" and
         "the Second Secure Option") by the second anniversary of the
         Commencement Date.

I.       BTI did not exercise the New Option by 1 July, 1996.

J.       As a result of the exercise of the option by Castella and Secure, the
         failure by BTI to exercise the First BTI Option, the failure by
         Castella to exercise the Second Castella Option, the failure by Secure
         to exercise the Second Secure Option and the issue of shares to BTI
         pursuant to the First Variation, the issued capital of the Company is
         owned in the following proportions:

         BTI:              30%

         Castella:         15%

         Secure:           55%

K.       BTI has agreed to provide an amount of equity capital to the Company
         and prior to the date of this Agreement, has subscribed that capital to
         the Company.

L.       In line with the intent of the Shareholders' Agreement that on the
         provision by BTI of equity capital to the Company such shares in the
         Company would be issued to maintain the shareholdings of BTI, Castella
         and Secure in the Company in the ratios set out in Recital J, the
         Shareholders have agreed to cause the Company to allot to BTI a certain
         addition number of "A" shares in the capital of the Company at a
         premium and to allot to Castella and Secure certain additional "B"
         Shares in the capital of the Company at par.

M.       The Shareholders and the Company have also agreed that the Company will
         allot to BTI, Castella and Secure fresh options on the terms set out in
         this Agreement.

N.       Clause 18.5 of the Shareholders' Agreement provides that the
         Shareholders' Agreement may not be modified, amended, added to or
         otherwise varied except by a document in

                                      -4-
<PAGE>

         writing signed by each of the parties or signed on behalf of each party
         by a director under hand.

O.       The parties wish by this Agreement to record the matters set out in
         Recitals K to M and to vary the Shareholders' Agreement to the extent
         necessary to take account of those matters.

IT IS AGREED

1.       DEFINITIONS AND INTERPRETATION
         ------------------------------

1.1      DEFINITIONS

         In this Agreement (including in the Recitals), unless the contrary
         intention appears, each defined word and expression has the meaning
         assigned to that word or expression in the Shareholders' Agreement.

1.2      INTERPRETATION

         Clauses 1.3 and 1.4 of the Shareholders' Agreement are incorporated in
         and form part of this Agreement as if each referenced to "THIS
         AGREEMENT" in those clauses were a reference to this Agreement to vary
         the Shareholders' Agreements.

1.3      RECITALS

         The parties acknowledge and agree that the Recitals are true and
         correct and accurately reflect the circumstances in which this
         Agreement was entered into.

2.       ISSUE OF SHARES
         ---------------

2.1      ISSUE OF SHARES TO BTI

         (a)      On the date of this Agreement BTI must subscribe for, and be
                  issued with 300 "A" Shares in the capital of the Company
                  issued at a price of $1,791.67 per share (being $1.00 par and
                  $1,790.67 premium per share) for a total consideration of
                  $537,500.00.

         (b)      The Company acknowledges that prior to the date of this
                  Agreement BTI has tendered and the Company has received
                  payment for the "A" Shares to be issued to it pursuant to this
                  Clause 2.1.

2.2      ISSUE OF SHARES TO CASTELLA

         On the date of this Agreement Castella must subscribe for and be issued
         with 150 "B" Shares in the capital of the Company issued at the par
         price of $1.00 per share for a total consideration of $550.00 payable
         on the date of this Agreement.

                                      -5-
<PAGE>

2.3      ISSUE OF SHARES TO SECURE

         On the date of this Agreement Secure must subscribe for and be issued
         with 550 "B" Shares in the capital of the Company issue at the par
         price of $1.00 per share for a total consideration of.$550.00 payable
         on the date of this Agreement.

3.       ISSUE OF OPTIONS
         ----------------

3.1      ISSUE OF OPTIONS TO BTI

         (a)      On the date of this Agreement, the Company must issue to BTI
                  an option (the Option Terms attaching) to subscribe for, fully
                  pay up and be issued with a further 300 "A" Shares, in the
                  capital of the Company for a total consideration of
                  US$2,071,343.75 at any time on or before 31 December, 1996
                  ("THE OPTION").

         (b)      If BTI exercises the option the Company will issue BTI 300 "A"
                  Shares in the capital of the Company at a price per share of
                  A$1.00 (being the par value) plus a premium per share
                  calculated in A$ as at the date of the exercise of the option.

3.2      ISSUE OF OPTION TO CASTELLA

         On the date of this Agreement, the Company must issue to Castella an
         option (with the Option Terms attaching) to subscribe for, fully pay up
         and be issued with a further 150 "B" Shares in the capital of the
         Company issued at a price of $1.00 per share for a total consideration
         of $150.00 at any time on or before 31 December, 1996.

3.3      ISSUE OF OPTION TO SECURE

         On the date of this Agreement, the Company must issue to Secure an
         option (the Options Terms attaching) to subscribe for, fully pay up and
         be issued with a further 550 "B" Shares in the capital of the Company
         issued at a price of $1.00 per share for a total consideration of
         $550.00 at any time on or before 31 December, 1996.

4.       VARIATION OF SHAREHOLDERS' AGREEMENT
         ------------------------------------

4.1      VARIATION

         In consideration of the mutual agreements made by each party under this
         Agreement, the parties agree, pursuant to cause 18.5 of the
         Shareholders' Agreement, to vary clause 2.5 of the Shareholders'
         Agreement to the extent necessary to give effect to the provisions of
         Clauses 2 and 3 of this Agreement.

4.2      CONFIRMATION OF SHAREHOLDERS' AGREEMENT

         The parties confirm that the terms and conditions of the Shareholders'
         Agreement (as varied by this Agreement) remain in full force and
         effect.

                                      -6-
<PAGE>

4.3      ACKNOWLEDGEMENTS

         Without limiting Clause 4.2 but for the removal of doubt, the parties
         acknowledge that:

         (a)      pursuant to clause 5.4(a)(i) of the Shareholders' Agreement,
                  all paragraphs of clause 5.3 of the Shareholders' Agreement
                  except paragraphs (e), (f), (p) and (v) ceased to operate on
                  the first anniversary of the Effective Date and nothing in
                  this Agreement will operate to or be construed as bring back
                  into effect those paragraphs of clause 5.3 of the
                  Shareholders' Agreement which have ceased to operate; and

         (b)      BTI did not exercise the second BTI Option and nothing in this
                  Agreement will be interpreted to mean that BTI did exercise
                  the Second BTI Option and the Shareholders' Agreement will be
                  construed and operate accordingly.

                                      -7-
<PAGE>

SIGNED AS AN AGREEMENT on the date first appearing

SIGNED by BIOTRANSPLANT INC.        )

by its director ELLIOT LEBOWITZ     )                       /s/ ELLIOT LEBOWITZ
                                                            -------------------
in the presence of:                 )

-------------------------------------
(Witness)

SIGNED by CASTELLA RESEARCH         )

PTY LTD by its director MAL BRANDON )                       /s/ MAL BRANDON
                                                            -------------------
in the presence of:                 )

-------------------------------------
(Witness)

SIGNED by SECURE SCIENCES PTY LTD   )

by its director PETER MOUNTFORD     )                       /s/ PETER MOUNTFORD
                                                            -------------------
in the presence of:                 )

-------------------------------------
(Witness)

SIGNED by STEM CELL SCIENCES        )

PTY LTD by its director MEL BRANDON )                       /s/ MEL BRANDON
                                                            -------------------
in the presence of:                 )

-------------------------------------
(Witness)

                                      -8-

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