NEITHER THESE SECURITIES NOR ANY SECURITIES  ISSUABLE UPON THE CONVERSION HEREOF
HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND THE RULES
AND REGULATIONS  PROMULGATED THEREUNDER (THE "1933 ACT"), OR THE SECURITIES LAWS
OF ANY STATE.  NEITHER THESE  SECURITIES  NOR ANY  SECURITIES  ISSUABLE UPON THE
CONVERSION HEREOF MAY BE OFFERED FOR SALE, SOLD OR OTHERWISE  TRANSFERRED UNLESS
THEY ARE REGISTERED UNDER THE 1933 ACT AND THE LAWS OF APPLICABLE STATES OR SUCH
OFFER, SALE OR TRANSFER IS EXEMPT FROM SUCH REGISTRATION.

                 7% CONVERTIBLE DEBENTURE DUE September 30, 1998

$___________                                                  September 30, 1996
Number ____

           FOR VALUE RECEIVED,  American Biogenetic  Sciences,  Inc., a Delaware
corporation (the "Company"),  hereby promises to pay to _______________________,
or  registered  assigns  (the  "Holder") on  September  30, 1998 (the  "Maturity
Date"),   the   principal   amount   of    ______________________________Dollars
($___________),  and to pay interest on the  principal  amount  hereof,  in such
amounts, at such times and on such terms and conditions as are specified herein.

ARTICLE 1.  Interest

           The Company shall pay interest on the unpaid principal amount of this
Debenture (this "Debenture") at the rate of seven percent (7%) per year, payable
quarterly  in  arrears  on the  last  day of each  calendar  quarter  commencing
September  30,  1996  until  the  principal  hereof  is paid in full or has been
converted;  provided  that if the  Holder  shall  have  failed to  present  this
Debenture  for  payment  following  the  Maturity  Date  pursuant  to Article 2,
interest shall cease to accrue on the Maturity Date.  Interest on this Debenture
shall accrue from the most recent date to which interest has been paid or, if no
interest has been paid, from the date of issuance. Interest shall be computed on
the basis of a 360-day year of twelve 30-day months. If the Holder shall convert
this  Debenture  during any quarter,  the Company shall pay to the Holder,  upon
conversion,  the  pro-rata  portion  of accrued  interest  payable  through  the
conversion date in the manner provided in Section 3.1 below in lieu of cash.

ARTICLE 2.  Method of Payment

           This  Debenture  must be  surrendered to the Company in order for the
Holder to receive  payment of the principal  amount hereof.  Except as otherwise
provided  herein,  the Company  shall pay the  principal of and interest on this
Debenture in United States dollars.  However,  the Company may pay any principal
and interest  payable by cash, by a check payable in such money. The Company may
draw a check for the  payment  of  interest  to the order of the  Holder of this
Debenture  and mail it to the  Holder's  address  as shown on the  Register  (as
defined in Section 7.2 below).  Interest and principal payments shall be subject
to withholding  under applicable  United States Federal Internal Revenue Service
Regulations.





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Page 2


ARTICLE 3.  Conversion

           SECTION 3.1.        Conversion Privilege

           (a)        The Holder of this Debenture  shall have the right, at its
option,  to convert this Debenture in integrals of $10,000  principal  amount or
the remaining  principal amount hereof if less than $10,000 into shares of Class
A Common Stock,  par value $0.001 per share, of the Company  ("Common Stock") at
any time which is before the close of business on the Maturity  Date,  except as
set forth in Section 3.1(c) below. The number of shares of Common Stock issuable
upon the  conversion  of this  Debenture is determined by dividing the principal
amount  hereof to be  converted  plus all  accrued and unpaid  interest  thereon
through the conversion date by the conversion price (as defined in paragraph (b)
of this  section  3.1  below) in effect on the  conversion  date (as  defined in
Section 3.2 below).  On  conversion,  no further  payment of or  adjustment  for
accrued  interest  shall be made in  addition to that  provided in the  previous
sentence.

           (b)        The conversion price is eighty-three  percent (83%) of the
current  market price of the Common Stock on the  conversion  date as determined
under Section 3.7 below; provided,  however, that in no event may the conversion
price exceed $8.00 per share (the  "Maximum  Conversion  Price") and in no event
may the conversion  price be less than $3.00 per share (the "Minimum  Conversion
Price").  In the event that, but for this Section 3.1(b),  the conversion  price
would have been below $3.00, the difference  between (a) the number of shares of
Common Stock that would have been issued at what the conversion price would have
been but for the Minimum  Conversion Price limitation  contained in this Section
3.1(b) multiplied by 100% of the current market price of the Common Stock on the
conversion date as determined  under Section 3.7 below,  minus (b) the number of
shares of Common Stock actually issued  multiplied by 100% of the current market
price of the Common Stock on the conversion date as determined under Section 3.7
below,  shall be paid to the  Holder in cash (or by check  drawn to the order of
the Holder). The Minimum Conversion Price and the Maximum Conversion Price shall
be proportionately adjusted from time to time to reflect stock dividends,  stock
splits and stock combinations.

           (c)        Notwithstanding  anything  to the  contrary  set  forth in
Section 3.1(a), this Debenture may not be converted until the earlier of 90 days
after the first  closing of the sale of  Debentures  by the  Company in the same
offering as this Debenture was sold or such time as the  registration  statement
which the Company is to file with the  Securities and Exchange  Commission  (the
"Commission"),  pursuant to Section  2(a) of that  certain  Registration  Rights
Agreement being entered into contemporaneously  herewith between the Company and
the initial Holder of this Debenture, for the resale of the shares issuable upon
conversion  of  this  Debenture,   is  declared  effective  by  the  Commission.
Commencing  with the earlier of (i) 90 days after the first  closing of the sale
of Debentures by the Company in the same offering as this  Debenture was sold or
(ii) upon the effectiveness of such registration statement, 25% of the principal
amount  of  this  Debenture  may be  converted,  with an  additional  25% of the
principal  amount of this  Debenture  becoming  convertible on each of the 30th,
60th and 90th days thereafter.





7% Convertible Debenture
Page 3


           Notwithstanding the foregoing,  in the event the Company proposes, by
dividend or otherwise, to make a distribution to the holders of its Common Stock
generally,  of cash, securities (other than in a stock dividend,  stock split or
stock  combination) or other assets, the Company shall so notify the Holders not
less than five (5) business  days prior to the record date of such  distribution
and the entire  principal  amount of this  Debenture  shall  become  immediately
convertible  (at the option of each such  Holder) into Common Stock as otherwise
set forth herein.

           (d)        In the event this  Debenture  remains  outstanding  on the
Maturity  Date,  the Company  shall have the option to convert  the  unconverted
portion of such Debenture into shares of Common Stock on such date in the manner
set forth in this  Section  3.1 or to repay  the  outstanding  principal  amount
hereof plus any accrued and unpaid interest.

           Section 3.2.  Conversion  Procedure.  To convert this  Debenture into
Common  Stock,  the Holder must (a) complete  and sign the Notice of  Conversion
attached  hereto,  (b)  surrender  the  Debenture  to the  Company,  (c) furnish
appropriate  endorsements and transfer documents if reasonably  requested by the
Company  and (d)  subject to Section  3.4,  pay any  transfer  or similar tax if
reasonably required by the Company. The date upon which the Company receives the
completed  Notice  of  Conversion  (by  mail,  facsimile  or  otherwise)  is the
conversion  date,  provided  that the Company shall not be required to deliver a
certificate  for  Common  Shares  unless  and until  the  Company  receives  the
Debenture.  The Company shall use its best efforts to deliver a certificate  for
the number of full shares of Common Stock  issuable  upon the  conversion  and a
check for any  fraction of a share in  accordance  with Section 3.3 below within
three (3) business days after receipt of the Notice of Conversion,  provided the
Company has received the Debenture  from the Holder and any other items that may
be required  pursuant to this Section.  The person in whose name the certificate
of Common Stock is to be registered  shall be treated as a shareholder of record
on and after the conversion date. If the Holder converts more than one Debenture
at the same time, the number of full shares  issuable upon the conversion  shall
be based on the total principal  amount of Debentures  converted by such Holder.
Upon surrender of a Debenture that is to be converted in part, the Company shall
issue to the Holder a new Debenture equal in principal amount to the unconverted
portion of the Debenture surrendered.

           SECTION  3.3.  Fractional  Shares.  The  Company  shall  not  issue a
fractional share of Common Stock upon the conversion of this Debenture. Instead,
the Company  shall pay in lieu of any  fractional  share the cash value  thereof
(rounded to the nearest  1/100th of a share) at the then current market price of
the Common Stock as determined under Section 3.7 below.

           SECTION  3.4.  . Taxes  on  Conversion.  The  Company  shall  pay any
documentary,  stamp or similar  issue or transfer tax due on the issue of shares
of Common Stock upon the conversion of this Debenture. However, the Holder shall
pay any such tax which is due because the shares are issued in a name other than
its name.

           SECTION 3.5.  Company to Reserve Stock. The Company shall reserve out
of its  authorized  but  unissued  Common Stock or Common Stock held in treasury
such  number of shares of Common  Stock to permit the  conversion  of all of the
then outstanding Debentures at the Minimum Conversion





7% Convertible Debenture
Page 4


Price. All shares of Common Stock which may be issued upon the conversion hereof
shall be fully paid and nonassessable.

           SECTION 3.6. Restrictions on Transfer.  This Debenture and the Common
Stock issuable upon the  conversion  hereof have not been  registered  under the
Securities  Act of 1933 (the  "Act") and this  Debenture  and the  Common  Stock
issuable upon the conversion of this Debenture may not be offered for sale, sold
or otherwise transferred unless such offer, sale or other transfer is registered
under the Act or is exempt from such registration.

           SECTION 3.7.  Current Market Price.

           (a)        In  Sections  3.1 and 3.3,  the current  market  price per
share of Common  Stock on any date is the  average of the  quoted  prices of the
Common  Stock  for five  consecutive  trading  days  ending on the  trading  day
immediately preceding the date in question.

           (b)        As used in this  Section  3.7, the term quoted price shall
mean (i) the closing bid prices thereof on any such trading date, as reported by
Bloomberg,  L.P. or (ii) in the event the Common  Stock is not  reported on such
system,  the fair market value of the Common Stock as determined by the Board of
Directors of the Company in its good faith judgment.

           SECTION 3.8. Mergers, Etc. If the Company merges or consolidates with
another corporation or sells or transfers all or substantially all of its assets
to another  person and the holders of the Common  Stock are  entitled to receive
stock,  securities  or property in respect of or in exchange  for Common  Stock,
then as a condition of such merger, consolidation, sale or transfer, the Company
and any such successor,  purchaser or transferee shall agree that this Debenture
may thereafter be converted on the terms and subject to the conditions set forth
above into the kind and amount of stock,  securities or property receivable upon
such merger,  consolidation,  sale or transfer  which the Holder would have been
entitled to receive had the Holder converted this Debenture immediately prior to
such merger, consolidation, sale or transfer and been the owner of the number of
shares of Common  Stock  into  which this  Debenture  might have been  converted
immediately before such merger, consolidation, sale or transfer.

           Section 3.9. Listing. The Company shall use its best efforts to cause
all Common Shares into which this Debenture may be converted to be listed on (i)
the National  Association  of  Securities  Dealers  Automated  Quotation  System
("NASDAQ") National Market System, (ii) a national securities exchange, or (iii)
the NASDAQ small-cap market.

ARTICLE 4.  Mergers

           The Company shall not  consolidate  or merge into, or transfer all or
substantially  all of its assets to, any person,  unless such person assumes the
obligations  of the Company  under this  Debenture  and  immediately  after such
transaction  no Event of Default  exists.  Any  reference  herein to the Company
shall refer to such surviving or transferee  corporation  and the obligations of
the Company shall terminate upon such assumption.






7% Convertible Debenture
Page 5


ARTICLE 5.  Reports

           The Company will mail to the Holder hereof at its address as shown on
the  Register a copy of any annual,  quarterly  or current  report that it files
with the Securities and Exchange  Commission  ("SEC")  promptly after the filing
thereof and a copy of any annual,  quarterly or other report or proxy  statement
that it gives to its shareholders generally at the time such report or statement
is sent to shareholders.  In the event the Company is no longer required to file
reports  with the SEC,  the  Company  shall  provide  the  Holder  with  reports
containing  substantially  the same  information  as would be  included  in such
reports if they were required.

ARTICLE 6.  Defaults and Remedies

           SECTION 6.1.  Events of Default.  An "Event of Default" occurs if (a)
the Company does not make the payment of the  principal of this  Debenture  when
the same  becomes due and payable at  maturity,  (b) the Company does not make a
payment of interest when such interest  becomes due and payable and such default
continues  for a period of 5 days  thereafter,  (c) the  Company  fails to issue
shares of Common Stock upon conversion, (d) the Company fails to comply with any
of its other  agreements in this  Debenture  and such failure  continues for the
period and after the notice  specified  below,  (e) the  Company  pursuant to or
within the meaning of any Bankruptcy Law (as hereinafter defined): (i) commences
a voluntary  case;  (ii) consents to the entry of an order for relief against it
in an involuntary  case;  (iii)  consents to the  appointment of a Custodian (as
hereinafter defined) of it or for all or substantially all of its property; (iv)
makes a general  assignment for the benefit of its creditors;  or (v) a court of
competent  jurisdiction enters an order or decree under any Bankruptcy Law that:
(A) is for relief  against the Company in an  involuntary  case;  (B) appoints a
Custodian of the Company or for all or substantially  all of its property or (C)
orders the  liquidation of the Company,  and the order or decree remain unstayed
and in effect for 60 days.  As used in this Section  6.1,  the term  "Bankruptcy
Law" means Title 11 of the United  States  Code or any similar  federal or state
law for the relief of debtors. The term "Custodian" means any receiver, trustee,
assignee,  liquidator or similar  official under any  Bankruptcy  Law. A default
under clause (d) above is not an Event of Default  until the holders of at least
25% of the aggregate  principal  amount of the Debentures (as defined in Section
7.1) notify the Company of such  default and the Company does not cure it within
5 days after the receipt of such notice, which must specify the default,  demand
that it be remedied and state that it is a "Notice of Default."

           SECTION  6.2.  Acceleration.  If an Event of  Default  occurs  and is
continuing,  the  Holder  hereof by  notice  to the  Company,  may  declare  the
principal of and accrued interest on this Debenture to be due and payable.  Upon
such  declaration,  the principal  and interest  hereof shall be due and payable
immediately.

           SECTION 6.3.  Waivers.  The holders of a majority in principal amount
of  the  Debentures  then  outstanding  may  waive  a  default  or  rescind  the
declaration  of an Event  of  Default  and its  consequences  except  that (1) a
default in the payment of  principal of or interest on any  Debenture,  or (2) a
default  in the  conversion  of any  Debenture  may only be waived by the Holder
affected thereby.





7% Convertible Debenture
Page 6



ARTICLE 7.  Registered Debentures

           SECTION 7.1.  Series.  This Debenture is one of a numbered  series of
Debentures  having an  aggregate  principal  amount of not more than  $9,000,000
which are  identical  except as to the  principal  amount  and date of  issuance
thereof.   Such   Debentures  are  referred  to  herein   collectively   as  the
"Debentures". The Debentures shall be issued in whole multiples of $10,000.

           SECTION 7.2. Record Ownership.  The Company shall maintain a register
of the  holders of the  Debentures  (the  "Register")  showing  their  names and
addresses and the serial numbers and principal  amounts of Debentures  issued to
or  transferred  of record  by it from time to time.  A Holder  may  change  its
address on the Register in  accordance  with the notice  procedures in Article 8
below.  The  Register  may  be  maintained  in  electronic,  magnetic  or  other
computerized  form. The Company may treat the person named as the Holder of this
Debenture  in the  Register as the sole owner of this  Debenture.  The Holder of
this  Debenture  is the  person  exclusively  entitled  to receive  payments  of
interest  on  this  Debenture,   receive  notifications  with  respect  to  this
Debenture, convert it into Common Stock and otherwise exercise all of the rights
and powers as the absolute owner hereof.

           SECTION 7.3.  Registration  of Transfer.  Transfers of this Debenture
may be  registered  on the  books of the  Company  maintained  for such  purpose
pursuant  to  Section  7.2  above  (i.e.,  the  Register).  Transfers  shall  be
registered  when this  Debenture  is  presented to the Company with a request to
register  the  transfer  hereof  and  the  Debenture  is  duly  endorsed  by the
appropriate  person,  reasonable  assurances are given that the endorsements are
genuine and effective,  and the Company has received evidence satisfactory to it
that such  transfer is rightful  and in  compliance  with all  applicable  laws,
including tax laws and state and federal securities laws. When this Debenture is
presented for transfer and duly transferred hereunder,  it shall be canceled and
a new Debenture  showing the name of the transferee as the record Holder thereof
shall be issued in lieu hereof.  When this Debenture is presented to the Company
with a  reasonable  request  to  exchange  it for an equal  principal  amount of
Debentures  of other  denominations,  the Company  shall make such  exchange and
shall cancel this Debenture and issue in lieu thereof  Debentures having a total
principal amount equal to this Debenture in the  denominations  requested by the
Holder  (but only in  multiples  of $10,000 or the  remaining  principal  amount
hereof if less than  $10,000).  The Company may charge a reasonable  fee for any
registration  of transfer or exchange  other than one  occasioned by a notice of
redemption or the conversion hereof.

           SECTION 7.4.  Worn and Lost  Debentures.  If this  Debenture  becomes
worn, defaced or mutilated but is still  substantially  intact and recognizable,
the  Company  or its agent may issue a new  Debenture  in lieu  hereof  upon its
surrender. Where the Holder of this Debenture claims that the Debenture has been
lost,  destroyed or wrongfully taken, the Company shall issue a new Debenture in
place of the original  Debenture if the Holder so requests by written  notice to
the Company  actually  received by the  Company  before it is notified  that the
Debenture  has  been  acquired  by a bona  fide  purchaser  and the  Holder  has
delivered  to the  Company an  indemnity  bond in such amount and issued by such
surety as the Company  deems  satisfactory  together  with an  affidavit  of the
Holder setting





7% Convertible Debenture
Page 7


forth the facts  concerning  such loss,  destruction or wrongful taking and such
other  information in such form with such proof or  verification  as the Company
may request.

ARTICLE 8.  Notices

           Except as otherwise  provided in this Debenture,  any notice which is
required or convenient  under the terms of this Debenture shall be duly given if
it is in writing  and (a)  delivered  in person (b) mailed by first  class mail,
postage  prepaid,  or (c) sent by private  overnight  courier  service  (such as
Federal  Express) and directed to the Holder of the  Debenture at its address as
it appears on the  Register  or if to the  Company  to its  principal  executive
offices.  Such  notice  shall be  effective,  when  personally  delivered,  upon
receipt, four business days after deposit with the United States Postal Service,
or the next  business  day after  being sent when so sent by  private  overnight
courier service, as the case may be.

ARTICLE 9.  Time

           Where this  Debenture  authorizes or requires the payment of money or
the performance of a condition or obligation on a Saturday or Sunday or a public
holiday,  or authorizes or requires the payment of money or the performance of a
condition or obligation within, before or after a period of time computed from a
certain date, and such period of time ends on a Saturday or a Sunday or a public
holiday,  such payment may be made or condition or  obligation  performed on the
next  succeeding  business day, and if the period ends at a specified hour, such
payment may be made or condition  performed,  at or before the same hour of such
next  succeeding  business  day,  with the same  force and  effect as if made or
performed in accordance with the terms of this Debenture. Where time is extended
by virtue of the  provisions  of this Article 9, such extended time shall not be
included in the computation of interest.

ARTICLE 10.  Rules of Construction

           In this Debenture,  unless the context otherwise  requires,  words in
the singular  include the plural,  and in the plural  include the singular,  and
words of the masculine gender include the feminine and the neuter,  and when the
sense so  indicates,  words of the neuter  gender may refer to any  gender.  The
titles of sections  contained in this Debenture are inserted for  convenience of
reference  only,  and they neither form a part of this Debenture nor are they to
be  used  in the  construction  or  interpretation  hereof.  Wherever,  in  this
Debenture,  a  determination  of  the  Company  is  required  or  allowed,  such
determination  shall be made by a  majority  of the  Board of  Directors  of the
Company and if it is made in good faith, it shall be conclusive and binding upon
the Company and the Holder of this Debenture.

ARTICLE 11.  Governing Law

           The validity,  terms,  performance  and enforcement of this Debenture
shall be governed and construed by the provisions  hereof and in accordance with
the laws of the State of New York  applicable to agreements that are negotiated,
executed, delivered and performed solely in the State of New York (except as the
corporate laws of the State of Delaware may mandatorily apply thereto).





7% Convertible Debenture
Page 8


ARTICLE 12.  No Recourse to Stockholders, Officers, Directors and Certain Others

           No  recourse  shall be had for the  payment  of the  principal  of or
interest upon this Debenture or for any claim based thereon or otherwise against
any incorporator,  stockholder,  officer,  director,  accountant or attorney, or
other agent of the Company,  past,  present or future,  whether by virtue of any
constitution,  statute, rule of law, enforcement of any assessment or penalty or
by a reason of any other matter, all such liability, by acceptance hereof and as
part of the  consideration  of the issue or  transfer  hereof,  being  expressly
waived by the Holder.

           IN WITNESS  WHEREOF,  the Company has duly executed this Debenture as
of the date first written above.

                                           American Biogenetic Sciences, Inc.


                                           By 
                                              -----------------------------
                                                   Josef C. Schoell
                                                   Vice President, Finance





7% Convertible Debenture
Page 9


                              NOTICE OF CONVERSION
         [To be completed and signed only upon conversion of Debenture]

           The  undersigned,  the Holder of this Debenture,  hereby  irrevocably
elects to exercise the right to convert it into shares (the "Shares") of Class A
common stock, par value $0.001 per share, of American Biogenetic Sciences,  Inc.
as set forth below:

The undersigned intends to promptly sell the Shares   [_] YES           [_] NO



[Complete if less than             ____________________Dollars ($__________)*___
all of principal amount              ($10,000 or integral multiples of $10,000)
is to be converted]

[Signature must be                 _____________________________________________
guaranteed by a commercial                   (Name of Holder of shares if 
bank or a member firm of                  different than the registered Holder
the New York Stock                           of Debenture) 
Exchange if registered holder
of stock differs from registered
Holder of Debenture)
                                   _____________________________________________
                                 (Address of Holder if different than address of
                                          registered Holder of Debenture)

                                   _____________________________________________
                                  (Social Security or EIN of Holder of shares if
                                        different than Holder of Debenture)

           * If the  principal  amount of the  Debenture to be converted is less
           than the entire  principal  amount  thereof,  a new Debenture for the
           balance of the  principal  amount  shall be returned to the Holder of
           the Debenture.

Date:________________                Sign:_____________________________________
                                        (Signature must conform in all respects
                                         to name of Holder shown on face of this
                                         Debenture)
Signature Guaranteed:





7% Convertible Debenture
Page 10


                               ASSIGNMENT OF NOTE



           The undersigned hereby sell(s) and  assign(s)  and  transfer(s)  unto

________________________________________________________________________________
                     (name, address and SSN or EIN of assignee)

__________________________________________________Dollars ($_________)___
(principal amount of Debenture, $10,000 or integral multiples of $10,000)

of principal amount of this Debenture together with all accrued interest hereon.


Date:________                Sign:____________________________________________
                                 (Signature must conform in all respects to name
                                 of  Holder  shown on face of  Debenture  and be
                                 guaranteed by a commercial  bank or member firm
                                 of the New York Stock Exchange.)


Signature Guaranteed: