THESE  SECURITIES HAVE NOT BEEN REGISTERED  UNDER THE SECURITIES ACT OF 1933, AS
AMENDED,  OR ANY STATE  SECURITIES LAWS, AND MAY NOT BE SOLD OR OFFERED FOR SALE
EXCEPT  PURSUANT TO AN EFFECTIVE  REGISTRATION  STATEMENT  AS TO THE  SECURITIES
UNDER  SAID ACT AND ANY  APPLICABLE  STATE  SECURITIES  LAWS OR  PURSUANT  TO AN
AVAILABLE EXEMPTION FROM SUCH REGISTRATION REQUIREMENTS.

No. [__]          [$______]

                       AMERICAN BIOGENETIC SCIENCES, INC.
                    5% CONVERTIBLE DEBENTURE DUE MAY 20, 2001

            THIS DEBENTURE (this  "Debenture") is one of a duly authorized issue
of  Debentures  of  AMERICAN  BIOGENETIC  SCIENCES,  INC.,  a  corporation  duly
organized and existing under the laws of the State of Delaware (the  "Company"),
designated as its 5%  Convertible  Debentures  Due May 20, 2001, in an aggregate
principal   amount  of  Four  Million  U.S.   Dollars  (U.S.   $4,000,000)  (the
"Debentures").

            FOR VALUE RECEIVED,  the Company promises to pay to [___] the holder
hereof, or its order (the "Holder"),  the principal sum of [___] on May 20, 2001
(subject  to  extension  as provided  herein,  the  "Maturity  Date") and to pay
interest on the principal sum outstanding from time to time under this Debenture
("Outstanding  Principal Amount"), at the rate of 5% per annum payable only upon
conversion or redemption of this  Debenture and when due (at stated  maturity or
otherwise).  Interest  is payable  only on the portion of this  Debenture  being
converted,  redeemed or repaid.  Interest  shall accrue daily  commencing on the
first  business day to occur after the date hereof and shall continue until (and
including) the earliest to occur of (i) the Maturity  Date,  (ii) only as to the
portion of this Debenture being redeemed,  the date of payment of the applicable
redemption price, (iii) only as to the portion of the Debenture being converted,
the Holder  Conversion Date (as hereinafter  defined) or (iv) the day payment in
full of the  principal  sum of this  Debenture  has been made.  The  interest so
payable will be paid to the person in whose name this  Debenture (or one or more
predecessor  Debentures)  is registered on the records of the Company  regarding
registration  and  transfers  of  the  Debentures  (the  "Debenture  Register");
provided,  however,  that  the  Company's  obligation  to a  transferee  of this
Debenture  arises only if such  transfer,  sale or other  disposition is made in
accordance  with  the  terms  and  conditions  of  the  Securities  Subscription
Agreement  dated as of May 20,  1998  between  the  Company  and the Holder (the
"Subscription  Agreement").  The principal of, and (if the Company elects to pay
same in cash or if same is  payable  in  connection  with a  redemption  of this
Debenture)  interest on, this  Debenture are payable in such coin or currency of
the United States of America  that,  as of the time of payment,  is legal tender
for payment of public and private  debts,  at the address last  appearing on the
Debenture  Register of the Company as designated in writing by the Holder hereof
from  time to  time.  Except  as  otherwise  provided  herein,  the  Outstanding
Principal Amount and interest due hereunder shall bear interest,  from and after
the occurrence and during the  continuance of a default  hereunder,  at the rate
equal to the  lower of  twenty  percent  (20%)  per  annum or the  highest  rate
permitted by law.







            This Debenture is subject to the following additional provisions:

            1.    Exchange.  The  Debentures,  in  minimum  principal  amount of
$50,000,  are exchangeable for an equal aggregate principal amount of Debentures
of different denominations, as requested by the Holder surrendering the same. No
service charge will be charged to the Holder for such registration,  transfer or
exchange.

            2.    [Intentionally Omitted]

            3.    Transfers.   This   Debenture  has  been  issued   subject  to
investment   representations  of  the  original  purchaser  hereof  and  may  be
transferred  or  exchanged  in the United  States  only in  compliance  with the
Securities Act of 1933, as amended (the "Act"),  and applicable state securities
laws.  Prior to due presentment for transfer of this Debenture,  the Company may
treat  the  person  in whose  name  this  Debenture  is duly  registered  on the
Company's  Debenture  Register as the owner  hereof for the purpose of receiving
payment as herein provided and all other purposes, whether or not this Debenture
be overdue, and the Company shall not be affected by notice to the contrary.

            4.    Definitions.  For purposes  hereof the  following  definitions
shall apply:

                  "Closing  Bid Price"  shall mean the  Closing Bid Price of the
Common Stock on any applicable trading day as reported by Bloomberg, L.P. or, if
Bloomberg shall cease to report such data, by a mutually  acceptable  nationally
recognized  service reporting such data or, if the parties fail to agree, by the
relevant market or The Wall Street Journal.

                  "Closing Date" shall mean the date of original issuance of the
Debenture.

                  "Common Stock" shall mean the Class A Common Stock,  par value
$.001 per share, of the Company.

                  "Common Stock  Valuation  Period" shall mean the period during
which an average market price, whether Closing Bid Price or otherwise,  is being
determined for any purpose under this Agreement.

                  "Conversion  Default"  shall  have the  meaning  set  forth in
Paragraph 10(b).

                  "Conversion  Notice"  shall  have  the  meaning  set  forth in
Paragraph 6(e).

                  "Conversion   Price"  shall  mean  the  lesser  of  the  Fixed
Conversion Price or the Variable Conversion Price.

                  "Conversion   Rate"  shall  have  the  meaning  set  forth  in
Paragraph 6(d).


                                       -2-





                  "Fixed Conversion Price" shall mean $1.9375.

                  "Holder  Conversion  Date" shall have the meaning set forth in
Paragraph 6(e)(ii).

                  "Initial   Outstanding   Principal   Amount"  shall  mean  the
Outstanding Principal Amount of this Debenture on the Closing Date.

                  "Market  Price  for  Shares of Common  Stock"  shall  mean the
lowest  reported  sales  price  of one  share  of  Common  Stock  on the date of
valuation.

                  "NASDAQ" shall mean the NASDAQ National Market System.

                  "Post-Default  Conversion" shall have the meaning set forth in
Paragraph 10(b).

                  "Registration  Rights  Agreement"  shall have the  meaning set
forth in the Subscription Agreement.

                  "Subscription  Agreement"  shall have the meaning set forth on
page 1 of this Debenture.

                  "Variable  Conversion Price" shall mean the product of (x) the
average  Closing Bid Price of the Common Stock  during the five (5)  consecutive
trading days  immediately  preceding the date of conversion  and (y)(i) from the
Closing Date through 180 days after the Closing Date,  0.87;  (ii) from 181 days
after the Closing Date through 270 days after the Closing Date, 0.86; (iii) from
271 days after the Closing Date through 365 days after the Closing  Date,  0.85;
and (iv) from 366 days after the Closing Date, 0.84.

                  "Warrants"  shall mean the Initial  Warrants,  Second Warrants
and Third Warrants (each as defined in the Subscription  Agreement) to be issued
by the Company in connection with the issuance of this Debenture.

            5.    Paragraph 5 Transactions. (a) If at any time: (i) there occurs
any merger,  consolidation or other business  combination of the Company with or
into any other corporation or other entity or person (whether or not the Company
is the surviving corporation) or there occurs any other corporate reorganization
or  transaction or series of related  transactions,  and as a result thereof the
shareholders of the Company  immediately prior thereto own in the aggregate less
than 50% of the voting power of the ultimate parent  corporation or other entity
surviving or resulting from such merger, consolidation,  reorganization or other
transaction  or (ii)  the  Company  transfers  all or  substantially  all of the
Company's  assets to another  corporation  or other  entity or person,  then the
Holders  of  this  Debenture  then  outstanding  may  participate  in  any  such
transaction  as a class with  common  stockholders  on the same basis as if this
Debenture  had been  converted  one day prior to the  earlier of (x) the date of
public  announcement of such transaction or (y) consummation of such transaction
or (iii) a  purchase,  tender or exchange  offer is made to and  accepted by the
holders of more than 50% of the voting power of all


                                       -3-





outstanding   shares  of  Common   Stock   immediately   prior   thereto   then,
notwithstanding  Paragraph  6(a) hereof,  the Holder's  right to convert 100% of
this  Debenture  shall become fully vested with the Holder  Conversion  Date (as
defined  below)  being deemed to be the earlier of (x) the day prior to the date
of public  announcement  of such event or (y) the day prior to  consummation  of
such  event.  Each of the  foregoing  items (i),  (ii) and (iii) is  referred to
hereinafter as a "Paragraph 5 Transaction."

                  (b)   Alternatively,  at the  option  of the  Holder  of  this
Debenture,  the Company shall redeem any outstanding  Debentures effective as of
the  effective  date of any  Paragraph  5  Transaction  and the Holder  shall be
entitled to receive a  redemption  price  equal to 115% of the then  Outstanding
Principal  Amount of the Debenture,  plus accrued  interest on this Debenture to
(and  including) the effective  date of the Paragraph 5 Transaction.  Subject to
complying  with  clause (c) below,  such  Holder  shall be entitled to make such
election at any time up to the effective  date of the  Paragraph 5  Transaction;
provided,  however,  at the  discretion of such Holder,  such Holder may, at any
time,  elect to convert  its  Debentures  into fully  paid,  validly  issued and
nonassessable shares of Common Stock in accordance with the terms of Paragraph 6
hereof,  for  such  number  of  shares  of  Common  Stock as  determined  by the
application  of the  Conversion  Rate so long as the  Company  has not  paid the
redemption price for such Debentures.

                  (c)   At least  fifteen  (15) days  prior to, but not prior to
the public  announcement of such transaction  (unless no public  announcement is
required  under  applicable  rules  and  regulations),  the  consummation  of  a
Paragraph 5  Transaction,  the Company shall deliver  written notice thereof via
facsimile and overnight courier to the Holder. In order to redeem this Debenture
pursuant to Paragraph 5(b), the Holder shall surrender this Debenture, by either
overnight  courier  or  2-day  courier,  sent to  reach  the  Company  prior  to
completion  of the  Paragraph  5  Transaction,  to the  principal  office of the
Company,  and shall give  written  notice in the form of  EXHIBIT 1 hereto  (the
"Redemption  Notice") by facsimile  (with the original of such notice  forwarded
with the  foregoing  courier) to reach the Company at least five  business  days
prior to completion of the Paragraph 5 Transaction  at such office to the effect
that  Holder  elects  to have  redeemed  a  specified  principal  amount of this
Debenture  (plus  accrued  interest  thereon).  The  Company  shall use its best
efforts to pay the applicable  redemption price upon completion of the Paragraph
5 Transaction  and receipt of this Debenture or a notice that such Debenture has
been lost (together with an agreement pursuant to Section 18 hereof), to Holder,
together with a certificate, certified by an appropriate officer of the Company,
setting  forth  the  calculation  of  the  redemption   price  (the  "Redemption
Certificate") and, if appropriate,  a Debenture  evidencing the principal amount
covered by the submitted  Debenture not submitted for redemption;  provided that
in the case  where  more  than one  holder  submits  Debentures  for  redemption
simultaneously and the Company is unable to redeem all Debentures  submitted for
such  redemption,  the Company shall redeem a principal  amount from each holder
who submitted  Debentures for redemption  equal to such holder's pro rata amount
based on the principal  amount of Debentures held by such holder relative to the
principal amount of Debentures held by all holders who submitted  Debentures for
redemption.  If the Company shall fail to redeem all of the Debentures submitted
for  redemption  (other than pursuant to a dispute as to the  calculation of the
applicable  redemption  price),  in  addition to any remedy each Holder may have
under this Debenture,  the Subscription  Agreement and the  Registration  Rights
Agreement, the applicable redemption price payable in respect of such unredeemed
Debentures  shall be  increased  by an amount  equal to 1.0% of the  Outstanding
Principal Amount for each 30 day period after the required date of


                                       -4-





payment of the redemption  price (in each case,  pro rated for partial  periods)
until such redemption price is paid in full.  Notwithstanding the foregoing,  in
the event of a dispute  as to the  determination  of the  applicable  redemption
price,  the  amount of the  redemption  price  that is not in  dispute  shall be
promptly paid to the Holder in accordance  with this Paragraph  5(c). The Holder
shall then be  entitled,  within  sixty  (60) days of receipt of the  Redemption
Certificate,  to submit such dispute to the American Arbitration Association for
resolution  according to the then applicable rules thereof,  which determination
shall be final and binding on all parties. If it shall be determined that Holder
shall be entitled to receive  additional  monies in respect of such  redemption,
the Company  shall  deliver to Holder such amount within three (3) business days
of receipt of written notice of such determination.  The cost of such proceeding
shall be  shared  50% by the  Holder  involved  in such  dispute  and 50% by the
Company,  except to the extent the arbitrator presiding over the arbitration may
determine that another allocation is equitable.

            5A.   Conversion at the Option of the Company.  At any time from and
after the second  anniversary  of the Closing Date,  the Company may require the
Holder to convert all or part of the then  Outstanding  Principal  Amount of the
Debenture  plus accrued  interest  thereon to the date fixed for conversion on a
date not less than  sixty (60) days after the  Holder's  receipt of notice  (the
"Mandatory  Conversion  Notice")  from the Company  requiring  such  conversion;
provided,  however,  that the Company may not require such conversion unless the
Closing Bid Price on the trading day immediately preceding the date on which the
Mandatory  Conversion  Notice is sent to the Holder is equal to or greater  than
$3.0625.

                  The Mandatory Conversion Notice shall be sent by facsimile and
overnight  courier to the  Holder at the  address of the Holder set forth in the
Debenture  Register.  The date fixed for  conversion  set forth in the Mandatory
Conversion  Notice  shall be, for all purposes  hereof,  deemed to be the Holder
Conversion  Date. The Conversion  Ratio shall be determined under Paragraph 6(d)
hereof.

            6.    Conversion  at the  Option of the  Holder.  The Holder of this
Debenture shall have the following conversion rights:

                  (a)   Holder's   Right   to   Convert.   This   Debenture   is
convertible, at the option of the Holder hereof, into fully paid, validly issued
and  nonassessable  shares of Common  Stock as  follows:  (i) 25% of the Initial
Outstanding  Principal  Amount may be converted into Common Stock commencing 120
days following the Closing Date; (ii) 50% of the Initial  Outstanding  Principal
Amount may be converted  into Common Stock  commencing  150 days  following  the
Closing  Date;  (iii) 75% of the  Initial  Outstanding  Principal  Amount may be
converted into Common Stock  commencing 180 days following the Closing Date; and
(iv) 100% of the Initial  Outstanding  Principal  Amount may be  converted  into
Common Stock commencing 210 days following the Closing Date (the foregoing items
(i)-(iv)  are  referred  to  hereinafter  as  the  "Conversion   Restrictions");
provided,  however,  if the  Company  issues or sells any  securities  which are
convertible  into  or  exchangeable  for its  Common  Stock  or any  convertible
security, or any warrants or other rights to subscribe for or to purchase or any
options for the  purchase of its Common  Stock (other than (w) shares or options
issued or which may be issued pursuant to Company


                                       -5-





employee,   officer,   director  or  consultant   stock  or  option  or  similar
equity-based compensation plans now or hereafter in effect, (x) securities which
are convertible  into or  exchangeable  for Common Stock,  options,  warrants or
rights  outstanding  on the Closing  Date  reflected in the  Company's  periodic
reports filed to date under the Securities Exchange Act of 1934, as amended (the
"Exchange Act Reports"),  and (y) to complete  payment of the purchase price for
Stellar Bio  Systems,  Inc. as  reflected  in the  Exchange Act Reports) and (z)
securities,  options,  warrants and rights issued in connection  with a business
acquisition,  joint venture,  licensing arrangement or other non-capital raising
purpose which issuances are of Restricted Securities (as such term is defined in
Rule 144 of the Act))  whether  singly or together  with other  securities  (the
foregoing issuances are collectively  referred to herein as "Further Issuances")
the  terms of  which  contain  conversion  restrictions  that in the  reasonable
judgment  of Holder are less  restrictive  to the  Holders  than the  Conversion
Restrictions, the Holder shall have an option, to be exercised by written notice
to the Company within thirty (30) days of the Company's  notice, as set forth in
the following sentence, of closing of such Further Issuances,  to have the terms
of the conversion  restrictions set forth in such Further  Issuances replace the
Conversion  Restrictions.  The Company shall provide  written  notice to Holders
promptly after the closing of such Further Issuances.

                  (b)   Conversion  Limit.  The Company shall not be required to
issue  more  than  an  aggregate  of  4,000,000  shares  of  Common  Stock  (the
"Conversion  Limit") upon  conversion of all  Debentures in accordance  with the
terms of the Debentures.  Upon reaching the Conversion  Limit, the Company shall
have the option of either (i)  issuing  additional  shares of Common  Stock upon
conversion of Debentures if  shareholder  approval has been obtained (or if such
shareholder  approval is not  required to comply  with  applicable  rules of the
applicable  market upon which the Common Stock is traded) or (ii) paying cash to
the Holder in an amount equal to the amount of principal  being  converted  plus
the product of (x) the number of shares of Common  Stock that would be otherwise
issuable  upon  conversion  of  Debentures  and (y) the  difference  between the
highest  sales  price of the  Common  Stock on the  date of  Conversion  and the
Conversion Price.

                  (c)   If this  Debenture is converted in part,  the  remaining
portion  of  this  Debenture  not so  converted  shall  remain  entitled  to the
conversion rights provided herein.

                  (d)   Conversion  Price  for  Holder  Converted  Shares.   The
Outstanding  Principal Amount of this Debenture that is converted into shares of
Common  Stock  shall be  convertible  into the number of shares of Common  Stock
which results from application of the following formula:

                                        P
                         ------------------------------
                                Conversion Price
   P = Outstanding Principal Amount of this Debenture submitted for conversion

The number of shares of Common Stock into which each $1,000  principal amount of
this  Debenture  hereto  may  be  converted  is  hereafter  referred  to as  the
"Conversion Rate."


                                       -6-





The  Company  shall pay all  accrued  but  unpaid  interest  on the  Outstanding
Principal Amount of this Debenture that is converted into shares of Common Stock
at the time of such conversion.  The Company may, at its option,  pay any or all
such accrued  interest  either (i) in cash or (ii) if such shares are registered
for resale under a registration  statement in accordance  with the  Registration
Rights Agreement,  in shares of Common Stock valued at the applicable Conversion
Price.  Promptly  after the Closing Date,  the Company shall deliver a notice to
the Holder specifying  whether such interest shall be paid in the future in cash
or Common Stock.  This notice shall remain in effect unless the Company delivers
a notice  changing the interest  payment  method at least ten (10) days prior to
any applicable Holder Conversion Date (as hereinafter defined).

                  (e)   Mechanics  of  Conversion.  (i) In order to convert this
Debenture  (in whole or in part) into full  shares of Common  Stock,  the Holder
shall surrender this Debenture by either overnight courier or 2-day courier,  to
the principal  office of the Company,  and shall give written notice in the form
of EXHIBIT 2 hereto (the "Conversion Notice") by facsimile (with the original of
such notice forwarded with the foregoing  courier) to the Company at such office
that  the  Holder  elects  to  convert  a  specified  principal  amount  of  the
Outstanding Principal Amount of this Debenture (plus accrued interest) specified
therein,  which such notice and  election  shall be  irrevocable  by the Holder;
provided, however, that the Company shall not be obligated to issue certificates
evidencing  the shares of the Common Stock issuable upon such  conversion  (plus
interest  thereon)  unless either this  Debenture is delivered to the Company as
provided  above, or the Holder notifies the Company that this Debenture has been
lost, stolen or destroyed and promptly executes an agreement pursuant to Section
18 hereof  reasonably  satisfactory to the Company to indemnify the Company from
any loss incurred by it in connection with such Debentures.

                  (ii)  The  Company  shall  use its best  efforts  to issue and
deliver,  (within  three  business  days after  delivery  to the Company of such
Debenture(s),  or after receipt of such agreement and  indemnification)  to such
Holder of  Debenture(s)  at the address of the  Holder,  or to its  designee,  a
certificate  or  certificates  for the number of shares of Common Stock to which
the Holder  shall be entitled  as  aforesaid  (plus  interest  thereon),  or, if
requested  by Holder and  available  to the  Company,  issue  such  shares in an
electronic  format (e.g. via DWAC) together with a calculation of the Conversion
Rate and a Debenture or Debentures  for the principal  amount of this  Debenture
not  submitted  for  conversion  (if such shares are  requested  by Holder to be
issued in an electronic format,  then the calculation of the Conversion Rate and
any such  Debenture or Debentures  shall be sent to the Holder at the address of
the Holder or its  designee).  The  effective  date of  conversion  (the "Holder
Conversion  Date") shall be deemed to be the date on which the Company  receives
by  facsimile  the  Conversion  Notice,  and the person or persons  entitled  to
receive  the  shares of Common  Stock  issuable  upon such  conversion  shall be
treated  for all  purposes  as the record  holder or  holders of such  shares of
Common Stock on such date.

            7.    Conversion   upon   Maturity.   At  the  Maturity   Date,  the
Outstanding Principal Amount of Debentures outstanding at such time plus (unless
the  Company  elects to pay same in cash) any accrued  interest  thereon to (and
including) the Maturity Date shall be automatically  converted into Common Stock
of the Company in  accordance  with the terms of Paragraph 6 of this  Debenture,
without  notice.  The  Company  shall use its best  efforts to issue and deliver
within three business days after delivery to the Company of this  Debenture,  or
after receipt of the agreement and indemnification described in


                                       -7-





Paragraph 18, to the Holder of the Debenture at the address of the Holder, or to
its designee,  a certificate or certificates  for the number of shares of Common
Stock to which the Holder shall be entitled  hereunder (plus interest  thereon),
or, if requested by the Holder and  available to the Company,  issue such shares
in an electronic  format (e.g.  via DWAC),  together  with a calculation  of the
Conversion  Rate (if such  shares  are  requested  by  Holder to be issued in an
electronic format,  then the calculation of the Conversion Rate shall be sent to
the Holder at the address of the Holder or its designee).  The person or persons
entitled to receive the shares of Common  Stock  issuable  upon such  conversion
shall be treated for all purposes as the record holder or holders of such shares
of Common Stock on the Maturity  Date.  In the event of  conversion  pursuant to
this  Paragraph 7, the  Maturity  Date shall be a "Holder  Conversion  Date" for
purposes of this Debenture.

            8.    Stock Splits; Dividends; Adjustments; Reorganizations.

                  (a)   Stock  Splits and  Combinations.  The Company  shall not
effect any stock split, subdivision or combination with an effective date within
five (5) trading  days of the  Maturity  Date.  If the Company  shall effect any
stock split,  subdivision or combination,  then the Conversion  Limit, the Fixed
Conversion  Price  and the  Company  Conversion  Threshold  shall  be  equitably
adjusted and if such events occur during any Common Stock Valuation Period, then
the  Variable  Conversion  Price shall be  equitably  adjusted  to reflect  such
events.

                  (b)   Certain Dividends and  Distributions.  The Company shall
not make, or fix a record date for the  determination of holders of Common Stock
entitled  to  receive a dividend  or other  distribution  payable in  additional
shares of Common  Stock with an  effective  date within five (5) trading days of
the Maturity Date.

                  (c)   Adjustments for Further Issuances. In the event that, at
any time or from time to time after the Closing Date, the Company  completes any
Further  Issuance,  the terms of which  contain a fixed  conversion  price  (the
"Alternate Fixed Conversion Price") that is less than the Fixed Conversion Price
contained  herein,  then from and after the time of such Further  Issuance,  the
Fixed Conversion Price,  shall be equal to the Alternate Fixed Conversion Price.
In the event that, at any time or from time to time after the Closing Date,  the
Company completes any Further Issuance,  the terms of which contain a conversion
formula (the  "Alternate  Floating  Conversion  Formula")  that differs from the
calculation for the Variable  Conversion  Price, then from and after the time of
such  issuance,  the Holder  shall have the option,  to be  exercised by written
notice to the Company  within thirty (30) days of the Company's  notice,  as set
forth in the following  sentence,  of the closing of such Further  Issuance,  to
elect to have the Alternate  Floating  Conversion  Formula  replace the Variable
Conversion  Price.  The Company shall provide written notice to Holders promptly
after the closing of such Further Issuance.

                  (d)   Adjustment    for    Reclassification,    Exchange   and
Substitution.  In the  event  that at any time or from  time to time  after  the
Closing Date, the Common Stock issuable upon the conversion of the Debentures is
changed into the same or a different number of shares of any class or classes of
stock, whether by recapitalization,  reclassification or otherwise (other than a
subdivision  or  combination  of  shares  or stock  dividend  or  reorganization
provided for elsewhere in this Paragraph 8 or


                                       -8-





a merger or  consolidation,  provided for in Paragraph 5), then and in each such
event each holder of Debentures  shall have the right thereafter to convert such
Debentures  into  the  kind of  stock  receivable  upon  such  recapitalization,
reclassification  or other  change of shares of Common  Stock,  all  subject  to
further  adjustment as provided  herein.  In such event,  the formulae set forth
herein for  conversion  shall be  equitably  adjusted to reflect  such change in
number of shares  or, if shares of a new class of stock are  issued,  to reflect
the market price of the class or classes of stock issued in connection  with the
above described transaction.

                  (e)   Reorganizations.  If at any  time or  from  time to time
after the Closing  Date there is a capital  reorganization  of the Common  Stock
(other than a recapitalization,  stock split, subdivision, combination, dividend
or  other  distribution   payable  in  additional  shares  of  Common  Stock  or
reclassification  provided for elsewhere in this Paragraph 8) then, as a part of
such  reorganization,  provision  shall  be  made  so that  the  Holders  of the
Debentures  shall  thereafter  be entitled  to receive  upon  conversion  of the
Debentures  the number of shares of stock or other  securities  or  property  to
which a holder  of the  number  of  shares  of  Common  Stock  deliverable  upon
conversion  of this  Debenture  would  have  been  entitled  upon  such  capital
reorganization.  In any such case,  appropriate  adjustment shall be made in the
application  of the provisions of this Paragraph 8 with respect to the rights of
the  Holders  of the  Debentures  after the  reorganization  to the end that the
provisions of this  Paragraph 8 shall be  applicable  after that event and be as
nearly equivalent as may be practicable,  including,  by way of illustration and
not  limitation,  by  equitably  adjusting  the  formulae  set forth  herein for
conversion to reflect the market price of the  securities or property  issued in
connection with the above described transaction.

                  (f)   Dispute.  In the event of a dispute  between a Holder of
this Debenture and the Company with respect to any of the  adjustments  required
pursuant to the  provisions of this  Paragraph 8, then the  Debentures  shall be
converted  in a manner  consistent  with the  latest  Schedule  of  Computations
delivered as set forth in  paragraph  (g) below.  Such Holder of this  Debenture
shall  then  be  entitled,  within  60  days  of  receipt  of  the  Schedule  of
Computations, to submit such dispute to the American Arbitration Association for
resolution according to then applicable rules thereof, which determination shall
be final and  binding.  If it shall be  determined  that a Holder of  Debentures
should have received additional shares of Common Stock upon such conversion (the
"Undelivered  Shares")  then,  within  three  trading days of receipt of written
notice of such determination, the Company shall issue to such Holder that number
of  additional  shares of Common  Stock as shall  have a value,  based  upon the
Market Price for Shares of Common Stock on the date of the receipt of the notice
of determination,  as shall equal the Undelivered  Shares times the Market Price
for  Shares of Common  Stock on the  Holder  Conversion  Date.  The cost of such
proceeding  shall be shared 50% by the Holder or Holders of Debentures  involved
in such  dispute  and 50% by the  Company,  except to the extent the  arbitrator
presiding  over  the  arbitration  may  determine  that  another  allocation  is
equitable.

                  (g)   Schedule of  Computations.  All adjustments  pursuant to
this  Paragraph 8 shall be  disclosed by notice in writing to the Holder of this
Debenture  within  three (3)  trading  days of the  occurrence  thereof and such
notice shall be accompanied by a Schedule of Computations  of such  adjustments.
If so requested by the Holder of this  Debenture,  the Company  shall provide to
such Holder


                                       -9-





within ten (10) trading days of its request  therefor a letter of concurrence to
the Schedule of  Computations  from the  independent  public  accountants of the
Company.

            9.    Fractional  Shares.  No  fractional  shares of Common Stock or
scrip  representing   fractional  shares  of  Common  Stock  shall  be  issuable
hereunder.  The  number of shares of Common  Stock  that are  issuable  upon any
conversion shall be rounded up or down to the nearest whole share.

            10.   Reservation of Stock Issuable Upon Conversion.

                  (a)   Reservation  Requirement.  The Company shall reserve and
keep available at all times,  free of preemptive  rights the number of shares of
Common  Stock equal to the  Conversion  Limit (less the number of shares  issued
upon  conversion  of  Debentures)  for the  purpose of  enabling  the Company to
satisfy any  obligation  to issue shares of its Common Stock upon  conversion of
all of the Debentures pursuant hereto.

                  (b)   Default.  If the  Company  does  not  have a  sufficient
number of shares of Common Stock available to satisfy the Company's  obligations
to a holder of  Debentures  upon receipt of a Conversion  Notice or is otherwise
unable to issue such shares of Common Stock in accordance with the terms of this
Agreement,  other than as set forth in Paragraph 6(b) hereof  (provided that the
Company is not in default of its  obligations  thereunder),  and such  condition
shall remain  unremedied for a period of forty-five (45) calendar days after the
Company's receipt of a Conversion Notice (a "Conversion Default"), then from and
after the fifth (5th) day following a Conversion Default (which for all purposes
shall be deemed to have occurred  upon the  expiration  of the  applicable  cure
period following the Company's receipt of the applicable Conversion Notice), the
Holder  of this  Debenture  shall  have the  right to  demand  from the  Company
immediate  redemption  of the  Debentures  in cash  at a  redemption  price  per
Debenture  equal to 120% of the  Outstanding  Principal  Amount of the Debenture
(including Debentures for which a Conversion Notice has not yet been sent), plus
accrued  interest  on the  Debenture;  provided,  however,  that  no  notice  of
redemption  may be  delivered by Holder  subsequent  to receipt by the Holder of
notice from the Company  (sent by overnight or 2-day courier with a copy sent by
facsimile)  of  availability  of  sufficient  shares of Common  Stock to perfect
conversion (a  "Post-Default  Conversion") of (subject to the Conversion  Limit)
all of the remaining outstanding Debentures; provided further that such right to
demand  redemption  shall be reinstated if the Company shall  thereafter fail to
perfect such Post-Default Conversion by delivery of Common Stock certificates in
accordance  with the applicable  provisions of Paragraph 6(e) hereof and payment
of all accrued and unpaid  interest  with respect  thereto  within five business
days of delivery of the notice of  Post-Default  Conversion.  In addition to the
foregoing,  upon a  Conversion  Default,  the  rate  of  interest  on all of the
Debentures  (including Debentures for which a Conversion Notice has not yet been
sent), shall, to the maximum extent of the law, be permanently  increased by two
percent (2%) per annum (i.e.,  from 5% to 7% per annum)  commencing on the first
day of the thirty  (30) day  period (or part  thereof)  following  a  Conversion
Default; an additional two percent (2%) per annum commencing on the first day of
each of the second and third such thirty (30) day periods (or part thereof); and
an  additional  one percent (1%) per annum on the first day of each  consecutive
thirty (30) day period (or part thereof)  thereafter  until such securities have
been duly  converted or redeemed as herein  provided;  provided that in no event
shall the rate of interest exceed the lower of 20% or the highest rate permitted
by

                                      -10-





applicable  law.  Any such  interest  which is not paid when due  shall,  to the
maximum  extent  permitted by law,  accrue  interest until paid at the rate from
time to time applicable to interest on the Debentures as to which the Conversion
Default has occurred.

            11.   No Reissuance  of  Debentures.  No Debentures  acquired by the
Company by reason of  redemption,  purchase,  conversion  or otherwise  shall be
reissued,  and all such Debentures  shall be retired.  No additional  Debentures
(other  than  the  initially  authorized  $4,000,000  of  Debentures)  shall  be
authorized or issued without the consent of at least seventy-five  percent (75%)
in interest of the holders of  Outstanding  Principal  Amount of the  Debentures
immediately prior to reaching such seventy-five percent (75%) consent.

            12.   No Impairment.  The Company shall not  intentionally  take any
action which would impair the rights and  privileges of the Debentures set forth
herein or the Holders thereof.

            13.   Rank of Subordinated  Debt.  Without the prior written consent
of not less than  seventy-five  percent  (75%) of the holders in interest of the
Outstanding Principal Amount of the Debentures,  the Company shall not issue any
subordinated  debt that is of senior rank to the Debentures in respect of rights
or  preferences  as to payments  upon  liquidation,  dissolution,  winding up or
otherwise of the Company.  Any  subordinated  debt of the Company that is issued
and  outstanding  immediately  prior to the  Closing  Date shall rank pari passu
with, or lower than, the Debentures.

            14.   Limitation  on Number of  Conversion  Shares.  Notwithstanding
anything to the contrary contained herein,  each Conversion Notice shall contain
a representation that, after giving effect to the shares of the Company's Common
Stock to be issued  pursuant  to such  Conversion  Notice,  the total  number of
shares of the  Company's  Common Stock deemed  beneficially  owned by the Holder
(excluding shares that might otherwise be deemed beneficially owned by reason of
the  conversion   right  in  the   Debentures   owned  by  the  Holder  and  its
"affiliates"),  together  with all shares of the  Company's  Common Stock deemed
beneficially  owned by the Holder's  "affiliates"  as defined in Rule 144 of the
Act, will not exceed 4.9% of the then total issued and outstanding shares of the
Company's Common Stock;  provided,  however,  that the Holder may elect to waive
the  restrictions set forth in this Section 14 upon not less than sixty-one (61)
days prior written notice to the Company.

            15.   Registration Suspension. In the event that at any time or from
time to time  any  registration  statement  with  respect  to the  Common  Stock
issuable upon conversion of this Debenture is suspended or trading in the Common
Stock on  NASDAQ is  suspended  for a period of time  ("Blackout  Period"),  the
Maturity  Date  hereunder  shall be extended for a period equal to 1.5 times the
number  of days in such  Blackout  Period.  Furthermore,  additional  provisions
pertaining to the suspension of effectiveness of such registration statement set
forth in Paragraph 5A of the Registration Rights Agreement,  shall be applicable
in the  event  of a  Blackout  Period,  and  are  specifically  incorporated  by
reference herein.

            16.   Obligations  Absolute.  No provision of this  Debenture  shall
alter  or  impair  the  obligation  of  the  Company,   which  is  absolute  and
unconditional,  to pay the principal of, and interest on, this  Debenture at the
time, place and rate, and in the manner, herein prescribed.


                                      -11-





            17.   Waivers of Demand,  Etc. The Company hereby  expressly  waives
demand and presentment  for payment,  notice of nonpayment,  protest,  notice of
protest,  notice of dishonor,  notice of  acceleration  or intent to accelerate,
bringing of suit and  diligence in taking any action to collect  amounts  called
for hereunder  and will be directly and primarily  liable for the payment of all
sums  owing  and to be owing  hereon,  regardless  of and  without  any  notice,
diligence,  act or omission as or with respect to the  collection  of any amount
called for hereunder.

            18.   Replacement Debentures.  In the event that any Holder notifies
the  Company  that  its  Debenture(s)  have  been  lost,  stolen  or  destroyed,
replacement  Debenture(s) identical in all respects to the original Debenture(s)
(except for registration  number and Outstanding  Principal Amount, if different
than that shown on the original  Debenture(s)) shall be issued by the Company to
the Holder,  provided  that the Holder  executes  and delivers to the Company an
agreement  reasonably  satisfactory to the Company to indemnify the Company from
any loss incurred by it in connection with such Debenture(s).

            19.   Payment of Expenses.  The Company agrees to pay all reasonable
expenses,  including  reasonable  attorneys'  fees, which may be incurred by the
Holder in enforcing  the  provisions of this  Debenture  and/or  collecting  any
amount due under this Debenture,  the Subscription Agreement or the Registration
Rights Agreement.

            20.   Defaults. The following shall constitute "Events of Default":

                  (a)   The  failure  of the  Common  Stock to be  listed on the
NASDAQ, The New York Stock Exchange,  Inc. or The American Stock Exchange,  Inc.
for a period of five (5) consecutive trading days; or

                  (b)   Trading in the Common  Stock is  suspended on the NASDAQ
for a period of five (5) consecutive trading days, other than as a result of the
suspension of trading in securities on such market in general; or

                  (c)   Any money judgment (including any arbitration award, but
only if reduced to a judgment,  but excluding any arbitration  award rewarded in
respect of such  proceedings  commenced  pursuant to  Paragraph  8(f)),  writ or
warrant of  attachment,  or similar  process in excess of Five Hundred  Thousand
Dollars  ($500,000)  in the  aggregate  shall be  entered or filed  against  the
Company,  its  subsidiaries or any of their properties or other assets and which
shall remain unpaid, unvacated,  unbonded or unstayed for a period of sixty (60)
days or in any event later than ten (10) days prior to the date of any  proposed
sale thereunder; or

                  (d)   The Company  refuses at any time to honor any Conversion
Notice  issued in  accordance  with (and  subject  to) the terms of  Paragraph 6
hereof and deliver shares pursuant thereto; or


                                      -12-





                  (e)   The Company shall default in the payment of (i) interest
on this  Debenture,  and such default shall  continue for five (5) business days
after the due date thereof,  or (ii) the  Outstanding  Principal  Amount of this
Debenture; or

                  (f)   Any of the  representations  or  warranties  made by the
Company  herein,  in  the  Subscription  Agreement,  or in  any  certificate  or
financial or other statements  heretofore or hereafter furnished by or on behalf
of the Company in connection  with the execution and delivery of this  Debenture
or the  Subscription  Agreement  shall be false or  misleading  in any  material
respect  at the time made and such  condition  (to the  extent  capable of being
cured)  shall  continue  uncured  for a period of ten (10)  business  days after
notice from the Holder of such condition; or

                  (g)   The  Company  shall  fail to  perform  or observe in any
material  respect  any  covenant or  agreement  in the  Subscription  Agreement,
Warrants or this Debenture,  including, without limitation, the failure to honor
any Conversion  Notice and deliver  shares  pursuant  thereto,  and such failure
shall continue  uncured for a period of ten (10) business days after notice from
the Holder of such failure; or

                  (h)   The  Company  shall (1) become  insolvent;  (2) admit in
writing its  inability to pay its debts  generally  as they mature;  (3) make an
assignment  for  the  benefit  of  creditors  or  commence  proceedings  for its
dissolution;  or (4) apply  for or  consent  to the  appointment  of a  trustee,
liquidator  or  receiver  for it or for a  substantial  part of its  property or
business; or

                  (i)   A trustee, liquidator or receiver shall be appointed for
the Company or for a  substantial  part of its property or business  without its
consent  and  shall  not  be  discharged  within  sixty  (60)  days  after  such
appointment; or

                  (j)   Any  governmental  agency  or  any  court  of  competent
jurisdiction at the instance of any governmental  agency shall assume custody or
control of the whole or any  substantial  portion of the properties or assets of
the Company and shall not be dismissed within sixty (60) days thereafter; or

                  (k)   Bankruptcy,  reorganization,  insolvency or  liquidation
proceedings or other proceedings,  or relief under any bankruptcy law or any law
for the relief of debt shall be  instituted  by or against the  Company  and, if
instituted  against the Company,  shall not be dismissed  within sixty (60) days
after such  institution or the Company shall by any action or answer approve of,
consent  to,  or  acquiesce  in any such  proceedings  or admit to any  material
allegations of, or default in answering a petition filed in any such proceeding.

                  Unless an Event of Default  shall have been  waived in writing
by the Holder (which waiver shall not be deemed to be a waiver of any subsequent
default),  at the  option of and  (except  in the case of clause  (k)  above) on
notice by the  Holder  and in the  Holder's  sole  discretion,  the  Holder  may
consider  this  Debenture  immediately  due and  payable,  without  presentment,
demand, protest or notice of any kind, all of which are hereby expressly waived,
anything herein or in any other instruments contained


                                      -13-





to the contrary  notwithstanding,  and the Holder may  immediately,  and without
expiration  of any period of grace,  enforce any and all of the Holder's  rights
and remedies provided herein or any other rights or remedies afforded by law. In
such event, this Debenture shall be redeemed at a redemption price equal to 125%
of the Outstanding  Principal Amount of the Debenture,  plus accrued interest on
this Debenture. In addition to the foregoing,  and notwithstanding any waiver by
Holder with respect to an Event of Default,  upon an Event of Default,  the rate
of  interest on this  Debenture,  shall,  to the  maximum  extent of the law, be
permanently  increased by two percent  (2%) per annum  (i.e.,  from 5% to 7% per
annum)  commencing  on the  first day of the  thirty  (30) day  period  (or part
thereof)  following  the Event of Default;  an  additional  two percent (2%) per
annum  commencing  on the first day of each of the second and third such  thirty
(30) day periods (or part  thereof);  and an additional  one percent (1%) on the
first  day of  each  consecutive  thirty  (30)  day  period  (or  part  thereof)
thereafter  until this  Debenture has been duly  converted or redeemed as herein
provided;  provided that in no event shall the rate of interest exceed the lower
of 20% or the highest rate permitted by applicable  law. Any such interest which
is not paid when due shall,  to the  maximum  extent  permitted  by law,  accrue
interest until paid at the rate from time to time  applicable to interest on the
Debentures as to which the Event of Default has occurred.

            21.   Savings  Clause.  In case any  provision of this  Debenture is
held by a court of competent  jurisdiction to be excessive in scope or otherwise
invalid or  unenforceable,  such provision shall be adjusted rather than voided,
if possible,  so that it is enforceable to the maximum extent possible,  and the
validity and  enforceability of the remaining  provisions of this Debenture will
not in any way be affected or impaired thereby.

            22.   Entire Agreement.  This Debenture, and the agreements referred
to in this Debenture  constitute the full and entire understanding and agreement
between the Company and the Holder with respect to the subject  hereof.  Neither
this  Debenture  nor any term  hereof  may be  amended,  waived,  discharged  or
terminated  other than by a written  instrument  signed by the  Company  and the
Holder.

            23.   Assignment,  Etc. The Holder may,  subject to compliance  with
the Subscription  Agreement and to applicable federal and state securities laws,
transfer  or assign  this  Debenture  or any  interest  herein  and may  pledge,
encumber or transfer  any of its rights or interest in and to this  Debenture or
any part hereof and, without limitation,  each assignee,  transferee and pledgee
(which may include any affiliate of the Holder) shall have the right to transfer
or assign its interest.  Each such  assignee,  transferee and pledgee shall have
all of the rights of the Holder under this  Debenture.  The Company agrees that,
subject to  compliance  with the  Subscription  Agreement,  after receipt by the
Company of written  notice of  assignment  from the Holder or from the  Holder's
assignee,  all  principal,  interest  and other  amounts  which  are  then,  and
thereafter  become,  due under this  Debenture  shall be paid to such  assignee,
transferee or pledgee at the place of payment  designated  in such notice.  This
Debenture  shall be binding upon the Company and its  successors and shall inure
to the benefit of the Holder and its successors and assigns.

            24.   No Waiver.  No failure on the part of the Holder to  exercise,
and no delay in exercising,  any right,  remedy or power hereunder shall operate
as a waiver thereof,  nor shall any single or partial  exercise by the Holder of
any right, remedy or power hereunder preclude any other or future


                                      -14-





exercise of any other right,  remedy or power.  Each and every right,  remedy or
power hereby granted to the Holder or allowed it by law or other agreement shall
be cumulative and not exclusive of any other, and may be exercised by the Holder
from time to time.

            25.   Miscellaneous. Unless otherwise provided herein, any notice or
other  communication  to a party  hereunder  shall be  sufficiently  given if in
writing and personally  delivered or sent by facsimile with copy sent in another
manner  herein  provided  or sent by  courier  (which for all  purposes  of this
Debenture  shall include  Federal  Express,  UPS or other  recognized  overnight
courier) or mailed to said party by certified mail, return receipt requested, at
its address provided for in the Subscription  Agreement or such other address as
either may designate  for itself in such notice to the other and  communications
shall  be  deemed  to have  been  received  when  delivered  personally,  on the
scheduled arrival date when sent by next day or 2-day courier service or if sent
by facsimile  upon receipt of transmittal  confirmation  or if sent by mail then
three  days  after  deposit in the mail.  Whenever  the sense of this  Debenture
requires,  words in the singular shall be deemed to include the plural and words
in the plural shall be deemed to include the singular.  If more than one company
is named  herein,  the  liability of each shall be joint and several.  Paragraph
headings  are for  convenience  only and shall not  affect  the  meaning of this
document.

            26.   Choice of Law and Venue;  Waiver of Jury Trial. THIS DEBENTURE
SHALL BE  CONSTRUED  UNDER THE LAWS OF THE STATE OF NEW YORK  WITHOUT  REGARD TO
PRINCIPLES  OF  CONFLICTS  OF LAW  (OTHER  THAN  SECTION  5-1401 OF THE NEW YORK
GENERAL  OBLIGATIONS LAW) OR CHOICE OF LAW (OTHER THAN SECTION 5-1401 OF THE NEW
YORK GENERAL  OBLIGATIONS LAW). The parties hereto hereby agree that all actions
or proceedings  arising  directly or indirectly  from or in connection with this
Debenture  shall be litigated only in the Supreme Court of the State of New York
or the  United  States  District  Court for the  Southern  District  of New York
located  in New York  County,  New  York.  The  parties  hereto  consent  to the
jurisdiction  and venue of the foregoing  courts and consent that any process or
notice  of  motion or other  application  to  either  of said  courts or a judge
thereof may be served  inside or outside  the State of New York or the  Southern
District of New York by registered mail, return receipt  requested,  directed as
provided in Section 25 (and  service so made shall be deemed  complete  five (5)
days after the same has been posted as aforesaid)  or by personal  service or in
such other  manner as may be  permissible  under the rules of said  courts.  The
parties  hereto  hereby waive any right to a jury trial in  connection  with any
litigation pursuant to this Debenture.

            27.   Rule 144.  With a view to making  available  to the Holder the
benefits of Rule 144  promulgated  under the Act ("Rule 144") and any other rule
or  regulation  of the SEC that may at any time  permit  the  Holder to sell the
Underlying  Stock of the Company  issuable  upon  conversion  or exercise of the
Debentures  and the  Warrants to the public  without  registration,  the Company
agrees to use its reasonable best efforts to:

                  (i)   make and keep  public  information  available,  as those
terms are understood and defined in Rule 144, at all times;


                                      -15-





                  (ii)  file with the SEC in a timely  manner  all  reports  and
other  documents  required  of the  Company  under  the Act  and the  Securities
Exchange Act of 1934, as amended (the "Exchange Act"); and

                  (iii) furnish to any Holder, forthwith upon request, a written
statement by the Company  (provided  true at the time) that it has complied with
the  applicable  reporting and filing  requirements  of the Act and the Exchange
Act, a copy of the most recent  annual or quarterly  report of the Company,  and
such other  reports and  documents so filed by the Company as may be  reasonably
requested to permit any such Holder to take  advantage of any rule or regulation
of the SEC permitting the selling of any such securities without registration.

            IN WITNESS  WHEREOF,  the Company has caused this  instrument  to be
duly executed by an officer thereunto duly authorized.

Dated:  May 20, 1998

                                          AMERICAN BIOGENETIC SCIENCES, INC.

                                          By:  /s/ Joseph C. Schoell
                                             --------------------------
                                             Name:  Josef C. Schoell
                                             Title: Vice President Finance - CFO

                                             Address:  1375 Akron Street
                                             Copiague, NY 11726
                                             Telephone:  516-789-2600
                                             Facsimile:  516-789-1669
ATTEST

  /s/ Timothy J. Roach
- -----------------------
Name:  Timothy J. Roach
Title: Secretary and Treasurer




                                      -16-





                                    EXHIBIT 1
                      (To be Executed by Registered Holder
                          in order to Redeem Debenture)

                                REDEMPTION NOTICE
                                       FOR
                    5% CONVERTIBLE DEBENTURE DUE MAY 20, 2001

Reference  is  made  to  5%  Convertible  Debentures  due  May  20,,  2001  (the
"Debentures") of American Biogenetic  Sciences,  Inc. (the "Company") No. [____]
in the  principal  amount  of [$ ].  In  accordance  with  and  pursuant  to the
Debentures, the undersigned is entitled to and hereby elects to have the Company
redeem the number of  Debentures  indicated  below by tendering  the  Debentures
specified  below as of the date specified  below.  If payment is to be made to a
person other than the registered Holder of this Debenture,  the undersigned will
pay all transfer  taxes payable with respect  hereto.  No fee will be charged to
the Holder for any redemption, except for transfer taxes, if any.

Date of Redemption:                   __________________________________________

Number of Debentures to be redeemed:  __________________________________________

Please confirm the following information:

Redemption Price:                     __________________________________________

Please  issue  any check  drawn on an  account  of the  Company  into  which the
Debentures  are  being  redeemed  in the  following  name  and to the  following
address:

      Pay to:                         __________________________________________
                                      
                                      __________________________________________

                                      __________________________________________

      Social Security
      or Tax Payer I.D. Number:       __________________________________________

      Telephone Number:               __________________________________________
      Facsimile Number:               __________________________________________

      Authorization:                  __________________________________________
                                      By:_______________________________________
                                      Title:____________________________________
      Dated:                          __________________________________________











                                    EXHIBIT 2
                      (To be Executed by Registered Holder
                         in order to Convert Debenture)

                                CONVERSION NOTICE
                                       FOR
                    5% CONVERTIBLE DEBENTURE DUE MAY 20, 2001

The undersigned,  as Holder of the 5% Convertible  Debenture Due May 20, 2001 of
AMERICAN BIOGENETIC SCIENCES, INC. (the "Company"),  No. [ ], in the outstanding
principal amount of U.S. [$ ] (the  "Debenture"),  hereby  irrevocably elects to
convert U.S. $_______ of the outstanding  principal amount of the Debenture into
shares of Class A Common Stock,  par value $.001 per share (the "Common Stock"),
of the Company  according to the  conditions  of the  Debenture,  as of the date
written below. The undersigned  hereby requests that share  certificates for the
Common Stock to be issued to the undersigned  pursuant to this Conversion Notice
be issued in the name of, and delivered to, the  undersigned  or its designee as
indicated  below.  If shares are to be issued in the name of a person other than
the registered  Holder of this Debenture,  the undersigned will pay all transfer
taxes payable with respect hereto.  No fee will be charged to the Holder for any
conversion, except for transfer taxes, if any.

The  undersigned  represents  that,  after  giving  effect to the  shares of the
Company's  Common Stock to be issued  pursuant to such  conversion  notice,  the
total number of shares of the Company's Common Stock deemed  beneficially  owned
by the  undersigned,  together  with all shares of the  Company's  Common  Stock
deemed  beneficially owned by the undersigned's  "affiliates" as defined in Rule
144 of the Act, will not exceed 4.9% of the total issued and outstanding  shares
of the Company's Common Stock.

Conversion Information:        NAME OF HOLDER:

                                               By:______________________________
                                               Print Name:
                                               Print Title:
                                               Print Address of Holder:



                                               Issue Common Stock to:  _________
                                               at: _____________________________
                                               _________________________________
                                               Social Security or
                                               Taxpayer I.D. Number
                                               Date of Conversion


Applicable Conversion Rate




                                       -2-