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                                                                     EXHIBIT 4.3

                          REGISTRATION RIGHTS AGREEMENT

         THIS REGISTRATION RIGHTS AGREEMENT, dated as of the ___ day of August,
2001, is entered into by and among AMERICAN BIOGENETIC SCIENCES, INC., a
Delaware corporation (the "Company"), and the investors listed on SCHEDULE A
hereto (individually, an "Investor," and collectively, the "Investors").

                                    RECITALS

         WHEREAS, certain of the Investors (the "Series A Holders") hold shares
of the Company's Series A Convertible Preferred Stock and Series A Warrants and
possess registration rights and other rights pursuant to a Registration
Agreement dated as of March 3, 2000 between the Company and the Series A Holders
(the "Prior Agreement"); and

         WHEREAS, the Series A Holders are holders of all of the "Registrable
Securities" of the Company (as defined in the Prior Agreement), and desire to
terminate the Prior Agreement and to accept the rights created pursuant hereto
in lieu of the rights granted to them under the Prior Agreement; and

         WHEREAS, each of the Investors is a party to that certain Securities
Purchase Agreement, dated as of the date hereof (the "Series B Securities
Purchase Agreement") among the Company and the Investors providing, inter alia,
for the purchase by certain of the Investors of the Company's Series B
Convertible Preferred Stock and Series B Warrants;

         WHEREAS, among the conditions to the consummation of the transactions
contemplated by the Series B Securities Purchase Agreement is the execution and
delivery of a Registration Rights Agreement providing certain registration
rights for the Investors; and

         WHEREAS, each of the parties hereto desires to set forth in a single
document the registration and certain other rights of the Investors;

         NOW, THEREFORE, in consideration of the mutual promises, covenants and
conditions hereinafter set forth and for other good and valuable consideration
the Series A Holders hereby agree that the Prior Agreement shall be superseded
and replaced in its entirety by this Agreement, and the parties hereto further,
hereby agree as follows:

         Section 1. Certain Definitions. As used in this Agreement, the
following terms shall have the following respective meanings:

         "Charter" means the Company's Restated Certificate of Incorporation, as
amended.

         "Closing Date" shall have the meaning set forth in Section 2.1 of the
Series B Securities Purchase Agreement.
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         "Common Stock" means (i) the Company's Class A Common Stock, $.001 par
value, as authorized on the date of this Agreement, (ii) any other capital stock
of any class or classes (however designated) of the Company, authorized on or
after the date hereof, the holders of which shall have the right, without
limitation as to amount, either to all or to a share of the balance of current
dividends and liquidating dividends after the payment of dividends and
distributions on any shares entitled to preference, and the holders of which
shall ordinarily, in the absence of contingencies or in the absence of any
provision to the contrary in the Company's Charter, be entitled to vote for the
election of a majority of directors of the Company (even though the right so to
vote may have been suspended by the happening of a contingency), and (iii) any
other securities into which or for which any of the securities described in (i)
or (ii) may be converted or exchanged pursuant to a plan of recapitalization,
reorganization, merger, sale of assets or otherwise.

         "Company Indemnified Person" means the Company, its directors, each of
its officers who have signed or otherwise participated in the preparation of the
registration statement, each underwriter of the Registrable Securities so
registered (including any broker or dealer through whom such of the shares may
be sold) and each Person, if any, who controls the Company within the meaning of
Section 15 of the Securities Act.

         "Exchange Act" means the Securities Exchange Act of 1934, or any
similar federal statute, and the rules and regulations of the SEC thereunder,
all as the same shall be in effect at the time.

         "Excluded Registration" means a registration under the Securities Act
covering shares issued or issuable solely (i) in connection with an acquisition
of another entity or business or (ii) upon the exercise of stock options or
pursuant to employee benefit plans, including registration statements on Form
S-4, S-8 or any successor form.

         "Holder" means any person owning or having the right to acquire
Registrable Securities or any assignee thereof.

         "Holder Indemnified Person" means the Holder and each underwriter of
the Registrable Shares (including their officers, directors, affiliates and
partners) so registered (including any broker or dealer through whom such shares
may be sold) and each Person, if any, who controls such Holder or any such
underwriter within the meaning of Section 15 of the Securities Act.

         "Investors" means the investors listed on Schedule A hereto who execute
a counterpart of this Agreement.

         "Liabilities" means for purpose of Sections 6 and 7 hereof any claims,
damages, losses, liabilities or expenses.

          "Material Adverse Effect" means (i) a material adverse effect on the
results of operations, business or financial condition of the Company, or (ii)
any material limitation upon the ability of the Company to perform its
obligations under, or upon the legality, validity or enforceability of, this
Agreement.

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         "Persons" means an individual, corporation, partnership, joint venture,
trust, or unincorporated organization, or a government or any agency or
political subdivision thereof.

         "Preferred Stock" means a collective reference to the Company's Series
A Convertible Preferred Stock and the Company's Series B Convertible Preferred
Stock.

         "Register", "registered", and "registration" refer to a registration
effected by preparing and filing a registration statement or similar document in
compliance with the Securities Act, and the declaration or ordering of
effectiveness of such registration statement or document.

         "Registrable Securities" means (i) Common Stock issuable or issued upon
conversion of the Preferred Stock, (ii) any Common Stock issuable or issued upon
the exercise of any of the Warrants and (iii) any Common Stock issued or
issuable with respect to the securities referred to in clauses (i) and (ii) by
way of a stock dividend or stock split or in connection with a combination of
shares, recapitalization, merger, consolidation or other similar event;
provided, however, that shares of Common Stock which are Registrable Securities
shall cease to be Registrable Securities upon any sale pursuant to a
registration statement under the Securities Act, Rule 144 promulgated under the
Securities Act, or any sale, transfer or assignment in any manner to any Person
who, by virtue of Section 14 or 15(d) hereof, is not entitled to the rights
provided by this Agreement.

         "Registration Statement" means a registration statement on Form S-3 (or
any similar form promulgated by the SEC which permits short form registration
using extensive incorporation by reference) filed under the Securities Act
pursuant to this Agreement; provided, however, that in the event the Company is
ineligible for Form S-3, "Registration Statement" shall mean any registration
statement under the Securities Act.

         "SEC" means the Securities and Exchange Commission, or any other
federal agency at the time administering the Securities Act and the Exchange
Act.

         "Securities Act" means the Securities Act of 1933, or any similar
federal statute, and the rules and regulations of the SEC thereunder, all as the
same shall be in effect at the time.

         "Series A Warrants" means those warrants issued pursuant to that
certain Securities Purchase Agreement, dated as of February 3, 2000 among the
Company and the Series A Holders.

         "Series B Warrants" means those warrants issued pursuant to the Series
B Securities Purchase Agreement.

         "Warrants" means, collectively, the Series A Warrants and the Series B
Warrants.

         Section 2. Demand Registrations.

                  (a) If, on or after ninety (90) days after the Closing Date,
at any time or from time to time, one or more Holders of at least 20% of the
Registrable Securities then outstanding shall notify the Company in writing (a
"Registration Notice") that it or they intend to offer or

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cause to be offered for sale or exchange Registrable Securities, the Company
will so notify (a "Registration Invitation") all Holders of Registrable
Securities, including all Holders who have a right to acquire Registrable
Securities. Within ten (10) days after receiving a Registration Invitation, each
Holder (other than the Holder or Holders giving the Registration Notice) shall
notify the Company in writing if such Holder requests that a Registration
Statement include all or any part of the such Holder's Registrable Securities.
Within thirty (30) days after the date the Company receives a Registration
Notice but not earlier than twenty (20) days thereafter, the Company will
prepare and file with the SEC a Registration Statement covering such of the
Registrable Securities as may be requested by any Holder thereof (including the
Holder or Holders giving the Registration Notice); provided, that, except with
respect to a Registration Statement for an offering to be made on a continuous
basis pursuant to Rule 415 of the Act (or any successor rule or similar
provision then in effect), the Company shall have no obligation to file a
Registration Statement covering Registrable Securities having an aggregate price
to the public (before deduction of any underwriters' discounts or commissions)
of less than $1,000,000. The Company shall use its reasonable best efforts to
cause each Registration Statement to be declared effective by the SEC within
sixty (60) days after the date such Registration Statement is filed with the
SEC. The Company shall use its best efforts to remain qualified to register
securities on Form S-3 (or any similar form promulgated by the SEC which permits
short form registration using extensive incorporation by reference) under the
Securities Act.

                  (b) The Company shall not be obligated to register, pursuant
to this Section 2, the Registrable Securities of any Holder who fails promptly
to provide the Company such information as the Company may reasonably request at
any time to enable the Company to comply with any applicable law or regulation
or to facilitate preparation of the Registration Statement.

                  Section 3. "Piggy-Back" Registrations.

                  (a) In addition to its rights under Section 2 hereof, if at
any time the Company shall determine to register (including for this purpose a
registration effected by the Company for stockholders other than the Holders)
any of its securities under the Securities Act, other than an Excluded
Registration, it shall send to each Holder of Registrable Securities, including
each Holder who has the right to acquire Registrable Securities, written notice
of such determination. If within ten (10) days after receipt of such notice,
such Holder shall so request in writing, the Company will, subject to the limits
of this Section 3, use its best efforts to include in the Registration Statement
all or any part of the Registrable Securities the Holder requests to be
registered therein.

                  (b) If the offering under this Section 3 involves an
underwriting of Common Stock to be issued by the Company and the managing
underwriter imposes a limitation on the number of shares of such Common Stock
which may be included in any such registration statement because, in its
judgment, such limitation is necessary to effect an orderly public distribution,
and such limitation is imposed pro rata with respect to all selling
stockholders, then the Company shall be obligated to include in such
registration statement only such limited portion of the Registrable Securities
which Holder has requested for inclusion hereunder as is so determined by the
underwriter.

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         Section 4. Registration Procedures.

                  (a) If and whenever the Company is required by the provisions
of this Agreement to effect the registration of Registrable Securities under the
Securities Act, the Company will:

                                    (i) maintain the effectiveness of the
                           Registration Statement until the earlier to occur of
                           (A) twenty-four (24) months after the date of
                           effectiveness of such Registration Statement
                           (excluding any period during which the Registration
                           Statement is suspended pursuant to Section 4(d)); (B)
                           the date on which the Registrable Securities are
                           sold; (C) the date that such shares cease to be
                           treated as Registrable Securities because all such
                           shares are eligible to be sold on a single day under
                           Rule 144 promulgated under the Securities Act; or (D)
                           the completion by the underwriters of the
                           distribution pursuant to such Registration Statement;

                                    (ii) prepare and file with the SEC such
                           amendments and supplements to the Registration
                           Statement and the prospectus used in connection
                           therewith as may be necessary to keep the
                           Registration Statement effective for the period set
                           forth in Section 4(a)(i) hereof;

                                    (iii) comply with the provisions of the
                           Securities Act with respect to the disposition of all
                           securities covered by the Registration Statement
                           during such period in accordance with the intended
                           methods of disposition by the sellers thereof set
                           forth in the Registration Statement;

                                    (iv) furnish to each seller of Registrable
                           Securities such number of copies of the Registration
                           Statement, each amendment and supplement thereto, the
                           prospectus included in the Registration Statement
                           (including each preliminary prospectus) and such
                           other documents as such seller may reasonably request
                           in order to facilitate the disposition of the
                           Registrable Securities owned by such seller;

                                    (v) use its best efforts to register or
                           qualify such Registrable Securities under such other
                           securities or blue sky laws of such jurisdictions as
                           any seller reasonably requests and do any and all
                           other acts and things which may be reasonably
                           necessary or advisable to enable such seller to
                           consummate the disposition in such jurisdictions of
                           the Registrable Securities owned by such seller
                           (provided that the Company shall not be required to
                           (i) qualify generally to do business in any
                           jurisdiction where it is not otherwise qualified or
                           otherwise would be required to qualify but for this
                           subparagraph, or (ii) consent to general service of
                           process in any such jurisdiction, unless it is
                           already subject to service in such jurisdiction);

                                    (vi) notify each seller of such Registrable
                           Securities, at any time when a prospectus relating
                           thereto is required to be delivered under the

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                           Securities Act, of the happening of any event as a
                           result of which the prospectus included in the
                           Registration Statement contains an untrue statement
                           of a material fact or omits any fact necessary to
                           make the statements therein not misleading, and,
                           subject to the right of the Company to suspend sales
                           under Section 4(d) below, at the request of any such
                           seller, the Company shall prepare a supplement or
                           amendment to such prospectus so that, as thereafter
                           delivered to the purchasers of such Registrable
                           Securities, such prospectus shall not contain an
                           untrue statement of a material fact or omit to state
                           any fact necessary to make the statements therein not
                           misleading;

                                    (vii) if the Registrable Securities are to
                           be sold in a firm commitment underwritten offering,
                           enter into a customary underwriting agreement and
                           take all such other actions as the holders of a
                           majority of the Registrable Securities being sold or
                           the underwriters reasonably request in order to
                           expedite or facilitate the disposition of such
                           Registrable Securities;

                                    (viii) advise each seller of such
                           Registrable Securities, promptly after it shall
                           receive notice or obtain knowledge thereof, of the
                           issuance of any stop order by the SEC suspending the
                           effectiveness of such Registration Statement or the
                           initiation or threatening of any proceeding for that
                           purpose and promptly use its best efforts to prevent
                           the issuance of any stop order or to obtain its
                           withdrawal if such stop order should be issued.

                                    (ix) cause all such Registrable Securities
                           registered pursuant hereunder to be listed on each
                           securities exchange or automated quotation system on
                           which similar securities issued by the Company are
                           then listed; and

                                    (x) permit, at the cost and expense of the
                           Company in accordance with Section 9 hereof, a single
                           firm of counsel designated as counsel of sellers of
                           Registrable Securities by the holders of a majority
                           in interest of the Registrable Securities to review
                           the Registration Statement and all amendments and
                           supplements thereto a reasonable period of time prior
                           to their filing with the SEC and state authorities,
                           and not file any such Registration Statement,
                           amendment or supplement in a form to which such
                           counsel reasonably objects.

         Section 5. Further Obligations of the Parties.

                  (a) Whenever under the preceding sections of this Agreement,
the Company is required hereunder to register Registrable Securities, it agrees
that it shall also do the following:

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                                    (i) Upon three days' prior written notice
                           and at reasonable times during normal business hours
                           and without undue interruption of the Company's
                           business or operations, permit each selling Holder or
                           his counsel or other representatives to inspect and
                           copy such corporate documents, records and properties
                           as may reasonably be requested by them to enable them
                           to exercise their due diligence responsibilities, and
                           cause the Company's officers, independent certified
                           public accountants and agents to supply any
                           information reasonably requested for that purpose;

                                    (ii) Furnish on the date that Registrable
                           Securities are delivered to the underwriters for sale
                           in connection with a registration pursuant to this
                           Agreement, if such securities are being sold through
                           underwriters, or, if such securities are not being
                           sold through underwriters, on the date that the
                           registration statement with respect to such
                           securities becomes effective, to each selling Holder
                           a copy of:

                                             (A) an opinion of counsel for the
                                    Company, dated the effective date of the
                                    Registration Statement, addressed to the
                                    underwriters, if any, and to the Holders
                                    requesting registration of Registrable
                                    Securities; and

                                             (B) a "comfort letter" signed by
                                    the Company's independent public accountants
                                    who have examined and reported on the
                                    Company's financial statements included in
                                    the Registration Statement, to the extent
                                    permitted by the applicable standards of the
                                    American Institute of Certified Public
                                    Accountants;

                  in each case covering substantially the same matters with
                  respect to the Registration Statement (and the prospectus
                  included therein) and with respect to events subsequent to the
                  date of the financial statements, as are customarily covered
                  in an opinion of issuer's counsel and in an accountants'
                  "comfort letter" delivered to the underwriters in underwritten
                  public offerings of securities in accordance with Statement on
                  Auditing Standards No. 72; and

                                    (iii) Use its best efforts to insure the
                           obtaining of all necessary approvals from the
                           National Association of Securities Dealers, Inc.

                  (b) Whenever the Holders are registering Registrable
Securities pursuant to any Registration Statement, each such Holder agrees to
timely provide to the Company, at its request, such information and materials as
it may reasonably request in order to effect the registration of such
Registrable Securities.

                  (c) Each Holder of Registrable Securities covenants and agrees
that (i) such Holder will not sell any Registrable Securities under a
Registration Statement until such Holder has received copies of the prospectus
as then amended or supplemented and notice from the Company that such
Registration Statement and any post-effective amendments thereto have

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become effective and (ii) such Holder and its officers, directors or affiliates,
if any, will comply with the prospectus delivery requirements of the Securities
Act as applicable to them in connection with sales of Registrable Securities
pursuant to such Registration Statement.

                  (d) Each Holder of Registrable Securities agrees that, upon
receipt of a notice from the Company of the occurrence of any event of the kind
described in Subsections 4(a)(vi) or 4(a)(viii), such Holder will forthwith
discontinue disposition of such Registrable Securities until such Holder's
receipt of the copies of the supplemented prospectus and/or amended Registration
Statement contemplated by Subsection 4(a)(iv), or until it is advised in writing
by the Company that the use of the applicable prospectus may be resumed, and, in
either case, has received copies of any additional or supplemental filings that
are incorporated or deemed to be incorporated by reference in such prospectus or
Registration Statement.


         Section 6. Indemnification of Holder Indemnified Persons.

                  (a) In the event that the Company registers any of the
Registrable Securities under the Securities Act, the Company will, to the extent
permitted by law, indemnify and hold harmless each Holder Indemnified Person
from and against any and all Liabilities, joint or several, to which they or any
of them become subject under the Securities Act or under any other statute or at
common law or otherwise, and, except as hereinafter provided, will reimburse the
Holder Indemnified Person, for any legal or other expenses reasonably incurred
by them or any of them in connection with investigating or defending any
actions, whether or not resulting in any Liability, insofar as such Liabilities
arise out of, or are based upon, any untrue statement or alleged untrue
statement of a material fact contained in the Registration Statement or any
filing with any state securities authority, in any preliminary or amended
preliminary prospectus or in the final prospectus (or the Registration Statement
or prospectus as from time to time amended or supplemented by the Company) or
arise out of or are based upon the omission or alleged omission to state therein
a material fact required to be stated therein or necessary in order to make the
statements therein not misleading, or any violation by the Company of any rule
or regulation promulgated under the Securities Act or any state securities laws
or regulations applicable to the Company and relating to action or inaction
required of the Company in connection with such registration, unless (i) such
untrue statement or alleged untrue statement or omission or alleged omission was
made in the Registration Statement, preliminary or amended preliminary
prospectus or final prospectus in reliance upon and in conformity with
information furnished in writing to the Company in connection therewith by such
Holder Indemnified Person expressly for use therein or unless (ii) in the case
of a sale directly by such Holder (including a sale of such Registrable
Securities through any underwriter retained by Holder to engage in a
distribution solely on behalf of Holder), such untrue statement or alleged
untrue statement or omission or alleged omission was contained in a preliminary
prospectus and corrected in a final or amended prospectus, and such Holder of
Registrable Securities failed to deliver a copy of the final or amended
prospectus at or prior to the confirmation of the sale of the Registrable
Securities to the Person asserting any such Liability in any case where such
delivery is required by the Securities Act or any state securities laws.

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                  (b) Promptly after receipt by any Holder Indemnified Person of
notice of the commencement of any action in respect of which indemnity may be
sought against the Company, such Holder Indemnified Person will notify the
Company in writing of the commencement thereof, and, subject to the provisions
hereafter stated, the Company shall assume the defense of such action (including
the employment of counsel, who shall be counsel reasonably satisfactory to such
Holder Indemnified Person and the payment of expenses insofar as such action
shall relate to any alleged Liability in respect of which indemnity may be
sought against the Company.

                  (c) Such Holder Indemnified Person shall have the right to
employ separate counsel in any such action and to participate in the defense
thereof, but the fees and expenses of such counsel shall not be at the expense
of the Company unless (i) the employment of such counsel has been specifically
authorized by the Company or (ii) representation by the counsel retained by the
Company would be inappropriate due to actual or potential differing interests
between such Holder Indemnified Person and any other party represented by such
counsel in such proceeding. The Company shall not be liable to indemnify any
Holder Indemnified Person for settlement of any action effected without the
Company's consent.

                  (d) The Company shall not, except with the approval of each
Holder Indemnified Person being indemnified under this Section, consent to entry
of any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by the claimant or plaintiff to the
parties being so indemnified of a release from all liability in respect to such
claim or litigation.

                  (e) If the indemnification provided for in this Section is
held by a court of competent jurisdiction to be unavailable to an indemnified
party with respect to any loss, liability, claim, damage, or expense referred to
therein, then the indemnifying party, in lieu of indemnifying such indemnified
party hereunder, shall contribute to the amount paid or payable by such
indemnified party as a result of such loss, liability, claim, damage, or expense
in such proportion as is appropriate to reflect the relative fault of the
indemnifying party on the one hand and of the indemnified party on the other in
connection with the statements or omissions that resulted in such loss,
liability, claim, damage, or expense as well as any other relevant equitable
considerations. The relative fault of the indemnifying party and of the
indemnified party shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of a material fact or the
omission to state a material fact relates to information supplied by the
indemnifying party or by the indemnified party and the parties' relative intent,
knowledge, access to information, and opportunity to correct or prevent such
statement or omission. Notwithstanding the foregoing, the liability of each
Holder under this Section shall be limited to an amount equal to the aggregate
proceeds (net of underwriting discounts and commissions) from the sale of the
Registrable Securities received by such Holder from the shares sold by such
Holder in the offering in question.

         Section 7. Indemnification of Company Indemnified Persons.

                  (a) In the event that the Company registers any of the
Registrable Securities under the Securities Act, each Holder of the Registrable
Securities so registered, to the extent permitted by law, will (severally and
not jointly) indemnify and hold harmless the Company

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Indemnified Persons from and against any and all Liabilities, joint or several,
to which they or any of them may become subject under the Securities Act or
under any other statute or at common law or otherwise, and, except as
hereinafter provided, will reimburse each such Company Indemnified Person for
any legal or other expenses reasonably incurred by them or any of them in
connection with investigating or defending any actions, whether or not resulting
in any Liability, insofar as such Liabilities arise out of or are based upon any
untrue statement or alleged untrue statement of a material fact contained in the
Registration Statement or any filing with any state securities commission or
agent, in any preliminary or amended preliminary prospectus or in the final
prospectus (or in the Registration Statement or prospectus as from time to time
amended or supplemented) or arise out of or are based upon the omission or
alleged omission to state therein a material fact required to be stated therein
or necessary in order to make the statements therein not misleading, but in each
case only to the extent that any such statement or omission was made in reliance
upon and in conformity with information furnished in writing to the Company
expressly for use in connection with such registration; provided, however, that
in no event shall any indemnity by a Holder under this Agreement exceed the
amount of the public offering price (net of underwriting discounts and
commissions) of all Registrable Securities offered by such Holder in the
offering question.

                  (b) Promptly after receipt of notice of the commencement of
any action in respect of which indemnity may be sought against such Company
Indemnified Person, the Company will notify such Holder in writing of the
commencement thereof, and such Holder shall, subject to the provisions
hereinafter stated, assume the defense of such action (including the employment
of counsel, who shall be counsel reasonably satisfactory to the Company) and the
payment of expenses insofar as such action shall relate to the alleged liability
in respect of which indemnity may be sought against the Holder.

                  (c) Each Company Indemnified Person shall have the right to
employ separate counsel in any such action and to participate in the defense
thereof, but the fees and expenses of such counsel shall not be at the expense
of such Holder of Registrable Securities unless employment of such counsel has
(i) been specifically authorized by such Holder of Registrable Securities or
(ii) representation by the counsel retained by such Holder would be
inappropriate due to actual or potential differing interests between the Company
and any other party represented by such counsel in such proceeding. Such Holder
of Registrable Securities shall not be liable to indemnify any Person for any
settlement of any such action effected without such Holder's consent.

                  (d) Such Holder shall not, except with the approval of each
party being indemnified under this Section, consent to entry of any judgment or
enter into any settlement which does not include as an unconditional term
thereof the giving by the claimant or plaintiff to the parties being so
indemnified of a release from all liability with respect to such claim or
litigation.

                  (e) If the indemnification provided for in this Section is
held by a court of competent jurisdiction to be unavailable to an indemnified
party with respect to any loss, liability, claim, damage, or expense referred to
therein, then the indemnifying party, in lieu of indemnifying such indemnified
party hereunder, shall contribute to the amount paid or payable

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by such indemnified party as a result of such loss, liability, claim, damage, or
expense in such proportion as is appropriate to reflect the relative fault of
the indemnifying party on the one hand and of the indemnified party on the other
in connection with the statements or omissions that resulted in such loss,
liability, claim, damage, or expense as well as any other relevant equitable
considerations. The relative fault of the indemnifying party and of the
indemnified party shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of a material fact or the
omission to state a material fact relates to information supplied by the
indemnifying party or by the indemnified party and the parties' relative intent,
knowledge, access to information, and opportunity to correct or prevent such
statement or omission. Notwithstanding the foregoing, the liability of each
Holder under this Section shall be limited to an amount equal to the aggregate
proceeds (net of underwriting discounts and commissions) from the sale of the
Registrable Securities received by such Holder from the shares sold by such
Holder in the offering in question.

         Section 8. Damages. The Company recognizes and agrees that the Holder
of Registrable Securities will not have an adequate remedy if the Company fails
to comply with this Agreement and that damages may not be readily ascertainable,
and the Company expressly agrees that, in the event of such failure, it shall
not oppose an application by the Holder of Registrable Securities or any other
Person entitled to the benefits of this Agreement requiring specific performance
of any and all provisions hereof or enjoining the Company from continuing to
commit any such breach of this Agreement.

         Section 9. Expenses of Registration.

                  (a) The Company shall bear and pay all expenses incurred in
connection with any registration, filing or qualification of Registrable
Securities with respect to the registrations pursuant to this agreement,
including (without limitation) all registration, filing, and qualification fees,
printers' and accounting fees, blue sky fees and expenses, including fees and
disbursements of counsel related to all blue sky matters, fees and expenses of
listing any Registrable Securities on any securities exchange or automated
quotation system on which shares of Common Stock are then listed, fees and
disbursements of counsel for the Company and the reasonable fees and
disbursements of one counsel for the selling Holders not to exceed $10,000;
provided, however, that the Company shall have no obligation to pay or otherwise
bear (i) any portion of the underwriters' commissions or discounts attributable
to the Registrable Securities, (ii) the cost and expense of procuring
underwriters' insurance in connection with the sale of Registrable Securities by
Holders of Registrable Securities, or (iii) any portion of the fees or
disbursements of more than one counsel for the selling Holders of Registrable
Securities in connection with the registration of their Registrable Securities.

                  (b) The Company shall pay all expenses in connection with any
registration initiated pursuant to Section 2 or 3 which is withdrawn, delayed or
abandoned at the request of the Company, unless such registration is withdrawn,
delayed or abandoned solely because of any actions of the Holders of Registrable
Securities.

                  (c) Notwithstanding paragraph (a) above, the Company shall not
be required to pay any expenses of a registration begun pursuant to Section 2 if
the request is subsequently

                                       11
<PAGE>   12
withdrawn at the request of the Holders of a majority of the Registrable
Securities covered thereby, unless at the time of withdrawal, the Holders have
learned of a Material Adverse Event with respect to the Company.

         Section 10. Right of Company To Delay Registration. For a period or
periods not exceeding ninety (90) days in the aggregate in any twelve (12) month
period, the Company shall not be obligated to prepare and file, or be prevented
from delaying, abandoning or suspending sales under, a Registration Statement
pursuant to this Agreement or any amendment to a Registration Statement or a
prospectus supplement thereunder at any time when the Company, in the good faith
judgment of the Company's Board of Directors upon advice of counsel, reasonably
believes:

                  (a) that the filing thereof at the time requested, or the
offering of Registrable Securities pursuant thereto, would materially and
adversely affect (i) a pending or scheduled public offering of the Company's
securities, (ii) an acquisition, merger, recapitalization, consolidation,
reorganization or similar transaction by the Company, (iii) pre-existing and
continuing negotiations, discussions or pending proposals with respect to any of
the foregoing transactions, or (iv) the financial condition of the Company in
view of the disclosure of any pending or threatened litigation, claim,
assessment or governmental investigation which may be required thereby; and

                  (b) that the failure to disclose any material information with
respect to the foregoing would cause a violation of the Securities Act or the
Exchange Act.

         Section 11. Conditions to Registration Obligations. The Company shall
not be obligated to effect the registration of Registrable Securities pursuant
to Section 2 or 3 unless all Holders of shares being registered consent to the
following conditions:

                  (a) conditions prohibiting the sale of shares by such Holders
until the Registration Statement shall have been effective for a specified
period of time;

                  (b) conditions requiring Holder to comply with all prospectus
delivery requirements of the Securities Act and with all anti-stabilization,
anti-manipulation and similar provisions of Section 10 of the Exchange Act and
any rules issued thereunder by the SEC, and to furnish to the Company
information about sales made in such public offering;

                  (c) conditions prohibiting such Holders upon receipt of
telegraphic or written notice from the Company (until further notice) from
effecting sales of shares, such notice being given to permit the Company to
correct or update a registration statement or prospectus;

                  (d) conditions requiring that at the end of the period during
which the Company is obligated to keep the Registration Statement effective
under Section 5 of the Securities Act, the Holders of shares included in the
Registration Statement shall discontinue sales of shares pursuant to such
Registration Statement upon receipt of notice from the Company of its intention
to remove from registration the shares covered by such Registration Statement

                                       12
<PAGE>   13
that remain unsold, and requiring such Holders to notify the Company of the
number of shares registered that remain unsold immediately upon receipt of
notice from the Company; and

                  (e) in connection with an offering made pursuant to Section 3,
conditions requiring the Holders of Registrable Securities to enter into an
underwriting agreement in form and substance reasonably satisfactory to the
Company, provided that any managing underwriter engaged by the Company shall
require the approval in writing of a majority of the Holders of Registrable
Securities requesting such registration, which consent shall not be unreasonably
withheld.

         Section 12. Transferability of Registration Rights. For all purposes of
this Agreement, the Holder of Registrable Securities shall include not only the
initial Investors but any general or limited partner of or any officer or
director of any Investor or their affiliates, including, but not limited to,
their immediate family, irrevocable trusts for estate planning purposes and
personal representatives; provided, however, that such assignee or transferee
agrees in writing to be bound by all of the provision of this Agreement

         Section 13. Limitation on Subsequent Registration Rights. From and
after the date of this Agreement, the Company shall not, without the prior
written consent of a majority of the Holders of the then outstanding Registrable
Securities, enter into any agreement with any holder or prospective holder of
any securities of the Company that would allow such holder or prospective holder
(a) to include such securities in any registration filed under this Agreement,
unless under the terms of such agreement, such holder or prospective holder may
include such securities in any such registration only to the extent that the
inclusion of such holder's securities will not in any manner reduce the amount
of the Registrable Securities of the Holders that is included or (b) exercise
any rights more favorable than those granted to the Holders pursuant to this
Agreement.

         Section 14. Miscellaneous.

                  (a) Notices. All notices, requests, consents and other
communications hereunder shall be in writing, shall be addressed to the
receiving party's address set forth in Section 9.1 of the Series B Securities
Purchase Agreement or to such other address as a party may designate by notice
hereunder, and shall be either (i) delivered by hand, (ii) made by telecopy or
facsimile transmission, (iii) sent by recognized national overnight courier
service, or (iv) sent by registered mail, return receipt requested, postage
prepaid. All notices, requests, consents and other communications hereunder
shall be deemed to have been given either (i) if by hand, at the time of the
delivery thereof to the receiving party at the address of such party set forth
above, (ii) if made by telecopy or facsimile transmission, on the next business
day after the time that receipt thereof has been acknowledged by electronic
confirmation or otherwise, (iii) if sent by overnight courier, on the next
business day following the day such notice is delivered to the courier service,
or (iv) if sent by registered mail, on the fifth business day following the day
such mailing is made.

                  (b) Entire Agreement. This Agreement, including exhibits, or
other documents referred to herein, embodies the entire agreement and
understanding between the

                                   13
<PAGE>   14
parties hereto with respect to the subject matter hereof and supersedes all
prior oral or written agreements and understandings relating to the subject
matter hereof. No statement, representation, warranty, covenant or agreement of
any kind not expressly set forth in this Agreement shall affect, or be used to
interpret, change or restrict, the express terms and provisions of this
Agreement.

                  (c) Amendments. Any term of this Agreement may be amended and
the observance of any term of this Agreement may be waived (either generally or
in a particular instance and either retroactively or prospectively), only with
the written consent of the Company and a majority of the Holders of Registrable
Securities then outstanding; provided, however that any amendment or waiver
having an adverse affect on any Holder shall only be effective if executed by
such Holder. Any amendment or waiver effected in accordance with this paragraph
shall be binding upon each holder of any Registrable Securities then
outstanding, each future holder of all such Registrable Securities, the
Investors, and the Company. No such waiver or consent, in either case, shall be
deemed to be or shall constitute a waiver or consent with respect to any other
terms or provisions of this Agreement, whether or not similar. Each such waiver
or consent, in either case, shall be effective only in the specific instance and
for the purpose for which it was given, and shall not constitute a continuing
waiver or consent.

                  (d) Assignment. Except as otherwise provided in Section 14
hereof, neither this Agreement nor any or all of the rights and obligations of a
party hereunder shall be assigned, delegated, sold, transferred or otherwise
disposed of by operation of law or otherwise, to any third person without the
prior written consent of the other party, and any attempted assignment,
delegation, sale, transfer, or other disposition, by operation of law or
otherwise, of this Agreement or of any rights or obligations hereunder contrary
to this Section 15(d) shall be void and without force or effect. Each party
shall be responsible for the compliance by its Affiliates with the terms and
conditions of this Agreement.

                  (e) Benefit. All statements, representations, warranties,
covenants and agreements in this Agreement shall be binding on the parties
hereto and shall inure to the benefit of the respective successors and permitted
assigns of each party hereto. Nothing in this Agreement shall be construed to
create any rights or obligations except among the parties hereto, and no person
or entity shall be regarded as a third-party beneficiary of this Agreement.

                  (f) Governing Law. This Agreement and the rights and
obligations of the parties hereunder shall be construed in accordance with and
governed by the law of the State of New York.

                  (g) Severability. In the event that any court of competent
jurisdiction shall determine that any provision, or any portion thereof,
contained in this Agreement shall be unreasonable or unenforceable in any
respect, then such provision shall be deemed limited to the extent that such
court deems it reasonable and enforceable, and as so limited shall remain in
full force and effect. In the event that such court shall deem any such
provision, or portion thereof, wholly unenforceable, the remaining provisions of
this Agreement shall be interpreted as if such provision were so excluded and
shall nevertheless remain in full force and effect.

                                        14

<PAGE>   15
                  (h) Headings and Captions. The headings and captions of the
various subdivisions of this Agreement are for convenience of reference only and
shall in no way modify, or affect the meaning or construction of any of the
terms or provisions hereof.

                  (i) No Waiver of Rights, Powers and Remedies. No failure or
delay by a party hereto in exercising any right, power or remedy under this
Agreement, and no course of dealing between the parties hereto, shall operate as
a waiver of any such right, power or remedy of the party. No single or partial
exercise of any right, power or remedy under this Agreement by a party hereto,
nor any abandonment or discontinuance of steps to enforce any such right, power
or remedy, shall preclude such party from any other or further exercise thereof
or the exercise of any other right, power or remedy hereunder. The election of
any remedy by a party hereto shall not constitute a waiver of the right of such
party to pursue other available remedies. No notice to or demand on a party not
expressly required under this Agreement shall entitle the party receiving such
notice or demand to any other or further notice or demand in similar or other
circumstances or constitute a waiver of the rights of the party giving such
notice or demand to any other or further action in any circumstances without
such notice or demand.

                  (j) Counterparts. This Agreement may be executed in one or
more counterparts, and by different parties hereto on separate counterparts,
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument.

                  (k) Further Assurances. In case at any time after the Closing
Date any further action is necessary or desirable to carry out the purposes of
this Agreement, the Company and the Investors will take such further action as
the other party may reasonably request, all at the sole cost and expense of the
requesting party (unless the requesting party is entitled to indemnification
therefor under Section 6 or 7).

                                        15

<PAGE>   16
         IN WITNESS WHEREOF, the undersigned have executed this Registration
Rights Agreement as of this ___ day of August, 2001.

                             AMERICAN BIOGENETIC SCIENCES, INC.


                             By:
                                    Name:    Josef C. Schoell
                                    Title:   President, COO and CFO


                             SERIES A HOLDERS:

                             -------------------------------------------
                             Alfred J. Roach

                             BIOTECHNOLOGY VALUE FUND, L.P.
                             By:      BVF PARTNERS L.P., its General Partner
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President

                             BIOTECHNOLOGY VALUE FUND II, L.P.
                             By:      BVF PARTNERS L.P., its General Partner
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President

                             INVESTMENT 10 L.L.C.
                             By:      BVF PARTNERS, L.P., its Investment Advisor
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President


                                        16

<PAGE>   17
                             SERIES B INVESTORS:

                             BIOTECHNOLOGY VALUE FUND, L.P.
                             By:      BVF PARTNERS L.P., its General Partner
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President

                             BIOTECHNOLOGY VALUE FUND II, L.P.
                             By:      BVF PARTNERS L.P., its General Partner
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President

                             INVESTMENT 10 L.L.C.
                             By:      BVF PARTNERS, L.P., its Investment Advisor
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President

                             BVF INVESTMENTS, L.L.C.
                             By:      BVF PARTNERS, L.P., its Investment Advisor
                                      By:      BVF, INC., its General Partner

                                               By:
                                                        ------------------------
                                                        Mark N. Lampert
                                                        President

                                        17

<PAGE>   18
                                   SCHEDULE A

                                    INVESTORS

Alfred J. Roach

BIOTECHNOLOGY VALUE FUND, L.P.

BIOTECHNOLOGY VALUE FUND II, L.P.

INVESTMENT 10 L.L.C.

BVF INVESTMENTS, L.L.C.



                              18