DEBENTURE

THIS DEBENTURE AND ANY SHARES  ISSUABLE UPON THE EXERCISE OF THIS DEBENTURE HAVE
NOT BEEN  REGISTERED  UNDER THE  SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"),
NOR UNDER ANY STATE SECURITIES LAW AND SUCH SECURITIES MAY NOT BE PLEDGED, SOLD,
ASSIGNED,  HYPOTHECATED,  OR  OTHERWISE  TRANSFERRED  EXCEPT AS PERMITTED BY THE
PROVISIONS OF REGULATION S UNDER THE ACT OR PURSUANT TO  REGISTRATION  UNDER THE
ACT OR AN AVAILABLE EXEMPTION FROM REGISTRATION.  THE TRANSFER OF THIS DEBENTURE
AND THE SHARES  ISSUABLE UPON THE EXERCISE HEREOF ARE RESTRICTED AS DESCRIBED IN
SECTION 7 HEREOF AND AS OTHERWISE DESCRIBED HEREIN.

No. D-01                                                            US$3,000,000



                               APHTON CORPORATION

              SERIES A CONVERTIBLE DEBENTURE DUE DECEMBER 19, 2007

     THIS  DEBENTURE  is a  duly  authorized  issue  of  APHTON  CORPORATION,  a
corporation  organized and existing under the laws of the State of Delaware (the
"Company")  designated as its Series A Convertible  Debenture due 2007. The date
on which this  Debenture  is issued is referred  to herein as the "Issue  Date."
This  Debenture  is issued by the  Company  pursuant to the  Debenture  Purchase
Agreement,  dated as of  December  __,  2002,  regarding  Series  A  Convertible
Debenture due 2007 of the Company (as such agreement may be amended from time to
time, the "Debenture Purchase Agreement").

     FOR VALUE RECEIVED, the Company promises to pay to [Aventis Pharmaceuticals
Inc.], a [Delaware]  corporation,  the registered  holder hereof (the "Holder"),
the principal sum of $3,000,000  (the "Face Amount") plus all accrued and unpaid
interest on December 19, 2007 (the  "Maturity  Date").  Interest shall accrue on
this Debenture as set forth herein.

     This Debenture is subject to the following additional provisions:

     1. INTEREST.  This Debenture shall bear interest at a rate of 11% per annum
on the unpaid Face Amount from the Issue Date until the Maturity  Date,  payable
in cash annually on January 5th of each year.  Interest will accrue daily on the
unpaid  principal  amount of this Debenture.  Upon the occurrence and during the
continuance of an Event of Default, interest will accrue on the unpaid principal
amount of this Debenture,  all accrued and unpaid interest on this Debenture and
any other amounts payable hereunder,  to the extent permitted by applicable law,
at 13% per annum.  In no event shall any interest  payable under this  Debenture
exceed the maximum rate of interest permissible under any applicable usury law.
<PAGE>


     2.  CONVERSION.  (a) The Holder is  entitled,  at its option after the date
that is 90 days after the date hereof,  subject to the  following  provisions of
this Section 2 and the necessary shareholder approvals as required by the Nasdaq
National Market or such other domestic  securities  exchange or quotation system
upon which shares of Common Stock,  par value $.001 per share ("Common  Stock"),
may be listed,  to  convert  all or a portion of this  Debenture  including  the
accrued and unpaid interest hereon (the "Conversion")  into unregistered  shares
of Common  Stock of the  Company,  at any time  until the  Maturity  Date,  at a
conversion price for each share of Common Stock (the  "Conversion  Price") equal
to the average closing price of Common Stock on the Nasdaq National Market,  the
Nasdaq  SmallCap  Market or the OTC Bulletin  Board, as the case may be, for the
five (5) trading days prior to the Conversion Date (as defined below);  provided
that (i) the principal amount being converted is at least US$100,000.00  (unless
if at the  time of  such  election  to  convert  the  principal  amount  of this
Debenture is less than US  $100,000.00,  then the whole amount thereof) and (ii)
the number of shares of Common  Stock  issuable  upon such  election to convert,
together  with any Common  Stock  already  owned by the Holder or any  affiliate
thereof, does not exceed 19.9% of the Common Stock outstanding as of the date of
the  Conversion  Date (the "19.9%  Cap").  The number of shares of Common  Stock
issuable upon exercise of this  Debenture (the  "Conversion  Shares") into which
this Debenture are convertible shall be determined by dividing (a) the principal
amount of this  Debenture to be converted,  together with all accrued and unpaid
interest hereon,  by (b) the Conversion Price then in effect.  In the event that
the number of Conversion  Shares exceeds the 19.9% Cap, the Company will pay the
remaining  principal  balance  of this  Debenture  plus all  accrued  and unpaid
interest hereon on the Maturity Date.

     (b) In order to convert  the  principal  amount of this  Debenture,  or any
portion thereof  (including any accrued and unpaid interest hereon),  the Holder
shall send by facsimile transmission (and confirm such transmission by telephone
or voicemail message) a notice of conversion (the "Notice of Conversion") to the
Company and the Company shall notify its transfer  agent,  U.S.  Stock  Transfer
Corporation,  located at 1745 Gardena Avenue, Glendale, California 91204-2991 or
any successor  thereto (the  "Transfer  Agent"),  stating  either (i) the dollar
amount or (ii) the  remaining  principal  amount  (and any  accrued  and  unpaid
interest hereon), to be converted and the applicable Conversion Price, and prior
to  Conversion,  the Holder must  physically  surrender  this  Debenture  to the
Company. In the event that the Holder converts less than the remaining principal
amount under this Debenture,  the amount to be converted shall first be deducted
from the accrued  and unpaid  interest,  and then the balance  shall be deducted
from the remaining principal amount.

     (c) No fractional shares of Common Stock or scrip representing fractions of
shares will be issued on conversion,  but the number of shares issuable shall be
rounded  down to the  nearest  whole  share.  The date on which  the  Notice  of
Conversion  is given (the  "Conversion  Date") shall be deemed to be the date on
which  the  Holder  faxes  or  otherwise  delivers  the  Notice  of  Conversion,
substantially  in the form annexed  hereto as Exhibit A, duly  executed,  to the
Company,  provided  that the Holder  shall  deliver to the Company the  original
Debenture being  converted  within five (5) business days thereafter (and if not
so delivered  within such time, the

                                      -2-
<PAGE>

Conversion Date shall be the date on which the later of the Notice of Conversion
and  the  original  Debenture  being  converted  is  received  by the  Company).
Facsimile  delivery of the Notice of Conversion shall be accepted by the Company
at facsimile number (305) 374-7615;  Attn: Chief Executive  Officer,  or at such
other  facsimile  number as the Company may provide to the Holder.  In the event
that (i) the entire  remaining  principal  amount  together with all accrued and
unpaid  interest under this Debenture is being converted in accordance with this
Section  2, such  fraction  shall be paid to the Holder "in cash" on the date of
such  conversion  and (ii)  less  than the  entire  remaining  principal  amount
together  with all accrued and unpaid  interest  under this  Debenture  is being
converted in accordance  with this Section 2, such fraction  shall be added back
to the principal amount remaining under this Debenture.

     3. REDEMPTION. (a) The Company is entitled, at its option at any time after
the date  hereof,  subject to the  following  provisions  of this  Section 3, to
redeem by payment to the Holder in cash or Conversion Shares or a combination of
both,  at the  Holder's  option,  the  principal  amount and  accrued and unpaid
interest of this Debenture as of the date of such redemption.

     (b) If (i) the  Company  and the  Holder  fail to  execute a  collaboration
agreement  between  the Company and the Holder by March 31, 2003 and (ii)(A) the
Company sells, licenses,  assigns,  pledges or otherwise transfers its rights to
the  Projects  (as  defined in the Letter of Intent  between the Company and the
Holder,  dated December __, 2002) or any other Company product to an entity that
is not the Holder or an  affiliate  thereof  or (B) a Change of Control  occurs,
then any portion of this  Debenture  that  remains  outstanding  as of that date
shall be, unless  otherwise  designated  by the Holder,  redeemed by the Company
within thirty (30) days by payment to the Holder in cash or Conversion Shares or
a combination  of both,  at the Holder's  option,  of the  principal  amount and
accrued and unpaid interest of this Debenture as of the date of such redemption.
"Change of Control" means the sale of the Company either by way of (i) a sale or
other  disposition of all or  substantially  all of the assets of the Company to
another  person or entity or (ii)  merger,  consolidation,  issuance  of shares,
reorganization,   share   exchange  or  any  other   transaction  or  series  of
transactions  to,  with or into  another  person or persons or entity in which a
single person or group of affiliated persons obtains ownership or control of 50%
or more of (A) the  voting  power or (B) any  class of equity  interests  of the
Company.

     (c)  If,  at  any  time  while  any  portion  of  this  Debenture   remains
outstanding,  the  Company  shall sell,  license,  assign,  pledge or  otherwise
transfer its rights to any of the Projects or any other  Company  product or any
other  rights  related  thereto  to any  entity  other  than the  Holder  or its
affiliates  or  subsidiaries,  then any portion of this  Debenture  that remains
outstanding  as of the  date  of  such  sale,  license,  assignment,  pledge  or
transfer,  shall be, unless otherwise designated by the Holder,  redeemed by the
Company  within  thirty (30) days by payment to the Holder in cash or Conversion
Shares or a combination of both, at the Holder's option, of the principal amount
and accrued and unpaid  interest of this  Debenture as of the date of such sale,
license, assignment, pledge or transfer.

     (d) Any  portion  of this  Debenture  that  remains  outstanding  as of the
Maturity Date shall be, unless otherwise  designated by the Holder,  redeemed by
the  Company  by  payment  to the

                                      -3-
<PAGE>


Holder in cash or Conversion  Shares or a  combination  of both, at the Holder's
option, of the principal amount of this Debenture  together with any accrued and
unpaid interest hereon on the Maturity Date.

     4. ADJUSTMENTS TO CONVERSION PRICE. (a) The number of Conversion Shares and
the  Conversion  Price  shall be  subject  to  adjustment  from  time to time as
provided in this Section 4. There shall be no adjustment  hereunder with respect
to (A) the  issuance  or sale of shares or  options  to  purchase  shares of the
Common Stock to employees,  officers,  directors and  consultants of the Company
and its  subsidiaries  (as such number of shares is  appropriately  adjusted for
subsequent   stock   splits,    stock   combinations,    stock   dividends   and
recapitalizations)  pursuant to plans or arrangements  approved by the Company's
Board  of  Directors;  (B) the  issue or sale to other  entities  or the  owners
thereof for acquisition  purposes;  and (C) the issue and sale to banks, savings
and loan associations,  equipment lessors or other similar lending  institutions
in connection with such entities  providing working capital credit facilities or
equipment financing to the Company.

     (b) The reclassification of Common Stock into securities (other than Common
Stock)  and/or  cash  and/or  other  consideration  shall be deemed to involve a
subdivision  or  combination,  as the case may be,  of the  number  of shares of
Common Stock  outstanding  immediately prior to such  reclassification  into the
number  or  amount  of  securities   and/or  cash  and/or  other   consideration
outstanding   immediately   thereafter,   and   the   effective   date  of  such
reclassification  shall be deemed  to be "the day upon  which  such  subdivision
becomes effective" or "the day upon which such combination  becomes  effective,"
as the case may be, within the meaning of Section 4(c).

     (c) In the  event  of  any  capital  reorganization  of  the  Company,  any
reclassification  of the stock of the Company  (other than a change in par value
or from par  value to no par  value  or from no par  value to par  value or as a
result of a stock dividend or  subdivision,  split-up or combination of shares),
any sale or transfer to another  person or entity of the property of the Company
as an entirety or substantially as an entirety,  or any  consolidation or merger
of the Company  with or into another  corporation  (where the Company is not the
surviving corporation or where there is a change in or distribution with respect
to  the  Common  Stock),   this  Debenture  shall  after  such   reorganization,
reclassification,  consolidation, transfer or merger be exercisable for the kind
and number of shares of stock or other  securities or property of the Company or
of the successor  corporation  resulting  from such  consolidation,  transfer or
surviving  such  merger,  if any, to which the holder of the number of shares of
Common Stock deliverable  (immediately prior to the time of such reorganization,
reclassification, consolidation or merger) upon exercise of this Debenture would
have been entitled upon such  reorganization,  reclassification,  consolidation,
transfer or merger.  The  provisions  of this clause  shall  similarly  apply to
successive  reorganizations,  reclassifications,  consolidations,  transfers, or
mergers.

     5.  GOVERNING  LAW.  This  Debenture  shall be governed by and construed in
accordance with the laws of the State of New York,  without giving effect to the
rules governing the conflicts of laws. The parties hereby irrevocably consent to
the jurisdiction of the courts of

                                      -4-
<PAGE>


the  State  of New  York  and of any  federal  court  located  in such  state in
connection  with any action or  proceeding  arising  out of or  relating to this
Debenture.

     6. EVENTS OF DEFAULT. The following shall constitute an "Event of Default":

     (a) The Company  fails to make any payment of principal or interest on this
Debenture as required  pursuant to this  Debenture and same shall continue for a
period of five (5) days; or

     (b) The Company  fails to issue  shares of Common Stock to the Holder or to
cause its Transfer  Agent to issue  shares of Common Stock upon  exercise by the
Holder of the  conversion  rights of the Holder in accordance  with the terms of
this Debenture, fails to transfer or to cause its Transfer Agent to transfer any
certificate  for shares of Common Stock issued to the Holder upon  conversion of
this  Debenture  and when  required  by this  Debenture,  and such  transfer  is
otherwise  lawful,  or fails to remove  any  restrictive  legend or to cause its
Transfer Agent to transfer any  certificate or any shares of Common Stock issued
to the Holder upon  conversion  of this  Debenture as and when  required by this
Debenture  and such legend  removal is  otherwise  lawful,  and any such failure
shall  continue  uncured for thirty (30) business days after written notice from
the Holder of such failure; or

     (c) The  Company  fails to redeem or cause to be redeemed  the  outstanding
portion of this  Debenture as and when required by the  provisions  set forth in
Section 3 and any such failure shall  continue  uncured for twenty (20) business
days after written notice from the Holder of such failure; or

     (d) The Company shall (1) make an  assignment  for the benefit of creditors
or commence proceedings for its dissolution;  or (2) apply for or consent to the
appointment  of a trustee,  liquidator  or receiver for its or for a substantial
part of its property or business; or

     (e) 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

     (f) 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

     (g) Bankruptcy,  reorganization,  insolvency or liquidation  proceedings or
other  proceedings for relief under any bankruptcy law or any law for the relief
of debtors  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 the material  allegations  of, or
default in answering a petition filed in any such proceeding; or

                                      -5-
<PAGE>


     (h) The Company is delisted from the Nasdaq  National  Market or the Nasdaq
SmallCap Market and is not quoted on the over-the-counter (OTC) market.

     Then, or at any time  thereafter  (i.e. the grace periods set forth above),
and in each and every such case,  unless  such Event of Default  shall have been
waived in writing by the Holder of this  Debenture  (which  waiver  shall not be
deemed to be a waiver of any subsequent default) at the option of the Holder and
in the discretion of the Holder, the Holder may

     (A) without notice or demand to the Company  declare all obligations of the
Company to the to be immediately due and payable;

     (B)  perform any  warranty,  covenant  or  agreement  which the Company has
failed to perform under this Agreement; and

     (C) pursue any available remedy to collect the principal of and interest on
this Debenture or to enforce the performance of any provision of this Debenture.

     7.  RESTRICTIONS  ON TRANSFER.  This Debenture shall also be subject to the
restrictions  on  transfer  set forth in Section 4.3 of the  Debenture  Purchase
Agreement.

     8. MISCELLANEOUS.  (a) Except as contemplated in the next sentence, nothing
contained in this Debenture shall be construed as conferring upon the Holder the
right to vote or to receive  dividends  or to  consent  or  receive  notice as a
stockholder in respect of any meeting of stockholders  or any rights  whatsoever
as a  stockholder  of  the  Company,  unless  and  to the  extent  converted  in
accordance  with the terms  hereof.  In the event the Company shall fix a record
date for the making of a distribution to holders of its Common Stock  (including
any such distribution made in connection with a consolidation or merger in which
the Company is the continuing  corporation),  of (i) assets (other than (A) cash
dividends or cash distributions or (B) dividends payable in Common Stock),  (ii)
evidences of indebtedness or other  securities  (except for Common Stock) of the
Company or of any entity other than the Company,  or (iii) subscription  rights,
options or warrants  to  purchase  any of the  foregoing  assets or  securities,
whether or not such rights, options or warrants are immediately exercisable (all
such distributions  referred to in clauses (i), (ii) and (iii) being hereinafter
collectively referred to as "Distributions on Common Stock"),  then, in any such
case,  the Company shall set aside,  in an escrow  reasonably  acceptable to the
Holder of this Debenture,  the Distributions on Common Stock to which the Holder
would have been entitled if the Holder had converted  this  Debenture for Common
Stock,  immediately  prior to the record  date for the  purpose  of  determining
stockholders entitled to receive such Distributions on Common Stock and any such
Distributions  on Common Stock (together with any earnings while escrowed) shall
thereafter  be  distributed  from time to time out of such  escrow to the Holder
upon  conversion of this Debenture or within five (5) business days  thereafter.
Upon the  payment  of this  Debenture  in cash on the  Maturity  Date,  any such
Distributions  on Common Stock set aside in escrow pursuant to this Section 8(a)
relating to this  Debenture or the portion  thereof being paid shall be released
and returned to the Company.

     (b) No  recourse  shall be had for the  payment  of the  principal  of this
Debenture,  or for

                                      -6-
<PAGE>


any  claim  based  hereon,   or  otherwise  in  respect   hereof,   against  any
incorporator,  stockholder or employee, as such, past, present or future, of the
Company or any successor Company, whether by virtue of any constitution, statute
or rule of law, or by the enforcement of any assessment or penalty or otherwise,
all  such  liability  being,  by  the  acceptance  hereof  and  as  part  of the
consideration for the issue hereof, expressly waived and released.

     (c) All payments  contemplated hereby to be made "in cash" shall be made in
immediately  available  good funds in such coin or currency of the United States
of America as at the time of payment is legal  tender for  payment of public and
private debts.  All payments of cash and each delivery of shares of Common Stock
issuable to the Holder as contemplated hereby shall be made to the Holder at the
address last appearing on this  Debenture or as otherwise  designated in writing
by the Holder from time to time; except that the Holder can designate, by notice
to the  Company,  a  different  delivery  address  for any one or more  specific
payments or deliveries.

     (e) Upon receipt by the Company of evidence of the loss, theft, destruction
or mutilation of this Debenture, and (in the case of loss, theft or destruction)
of  indemnity  or security  reasonably  satisfactory  to the  Company,  and upon
surrender and  cancellation of this Debenture,  if mutilated,  the Company shall
execute and deliver to the Holder a new  debenture  identical in all respects to
this Debenture.

     (f) No delay on the part of any party in  exercising  any  right,  power or
privilege  hereunder shall operate as a waiver thereof,  nor shall any waiver on
the part of any party of any right,  power or privilege  hereunder  preclude any
other or further exercise  thereof or the exercise of any other right,  power or
privilege hereunder.

     (g) All the  covenants,  stipulations,  promises  and  agreements  by or on
behalf of the Company  contained  in this  Debenture  shall be binding  upon the
Company's successors and assigns whether or not so expressed.

     (h) If any  provision  of this  Debenture  is found by a court of competent
jurisdiction to be invalid,  illegal or  unenforceable,  all other provisions of
this Debenture shall remain in effect,  and if any provision is in applicable to
any person or circumstances, such provision shall nevertheless remain applicable
to all other persons and circumstances.

     (i) The headings used in this  Debenture are for  convenience  of reference
only and do not define or limit the provisions hereof.

     (j) All notices,  requests and other  communications  hereunder  must be in
writing and will be deemed to have been duly given only if delivered  personally
against written receipt or by facsimile  transmission or mailed by prepaid first
class  certified  mail,  return  receipt  requested,  or  mailed  by  nationally
recognized  overnight courier prepaid, to the parties at the following addresses
or facsimile numbers:

                                      -7-
<PAGE>

                           If to the Registered Holder, to:

                           [Aventis Pharmaceuticals Inc.]
                           200 Crossing Boulevard
                           P.O. Box 6890
                           Bridgewater, NJ 08807-0890
                           Facsimile No.:
                           Attn:  [______________]


                           with a copy to:

                           Morgan, Lewis & Bockius LLP
                           101 Park Avenue
                           New York, NY  10178
                           Facsimile No.:  (212) 309-6273
                           Attn:  Randy Sunberg, Esq.

                           If to the Company, to:

                           Aphton Corporation
                           80 SW Eighth Street
                           Miami, FL  33130
                           Facsimile No.:
                           Attn:

                           with a copy to:

                           White & Case LLP
                           1155 Avenue of the Americas
                           New York, NY  10036
                           Facsimile No.:  (212) 354-8113
                           Attn:  Jonathan E. Kahn, Esq.

     All such notices,  requests and other  communications will (i) if delivered
personally  to the  address as provided in this  Section,  be deemed  given upon
delivery, (ii) if delivered by facsimile transmission to the facsimile number as
provided in this Section,  be deemed given upon  receipt,  (iii) if delivered by
mail in the manner  described  above to the address as provided in this Section,
be deemed given upon the earlier of the third business day following  mailing or
upon receipt and (iv) if delivered by nationally recognized overnight courier to
the address as provided in this  Section,  be deemed given on the earlier of the
first  business day  following the date sent by such  overnight  courier or upon
receipt  (in each case  regardless  of  whether  such  notice,  request or other
communication  is received by any other  person to whom a copy of such notice is
to be  delivered  pursuant  to this  Section).  Any party  from time to time may
change its address,  facsimile  number

                                      -8-
<PAGE>


or other  information  for the purpose of notices to that party by giving notice
specifying such change to the other party hereto.

     (k) All shares of Common Stock which are issuable  upon  conversion of this
Debenture  shall,  when  issued,  be duly and  validly  issued,  fully  paid and
nonassessable.  The Company  shall take all such  actions as may be necessary to
ensure that all such shares of Common Stock may be so issued  without  violation
of any  applicable law or  governmental  regulation or any  requirements  of any
domestic  securities  exchange or  quotation  system upon which shares of Common
Stock may be listed  (except  for  official  notice of  issuance  which shall be
immediately delivered by the Company upon each such issuance).

     (l) The  Company  shall  within  twelve  months  from the Issue Date hold a
meeting of its shareholders at which it shall seek shareholder  approval for the
issuance of the Common Stock issuable upon conversion of this Debenture.

                                      -9-
<PAGE>


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

Dated:  December ____, 2002

                                            APHTON CORPORATION


                                        By:  /s/ Philip C. Gevas
                                           -------------------------------------
                                            Name:  Philip C. Gevas
                                            Title:  Chief Executive Officer




ACKNOWLEDGED AND AGREED

By:  Aventis Pharmaceuticals Inc.



By:  /s/ Michael A. Yeomans
   --------------------------------
    Name:  Michael Yeomans
    Title:  President, Global
            Business Development
<PAGE>


                                                                       EXHIBIT A

                              NOTICE OF CONVERSION

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



     The undersigned hereby irrevocably elects to convert [$ ________________ of
the above Debenture No. ___] [the remaining principal amount,  together with all
accrued and unpaid interest thereon of the above Debenture No. ___,] into shares
of  Common  Stock  of  APHTON  CORPORATION  (the  "Company")  according  to  the
conditions hereof, as of the date written below.


Conversion Date*: _______________________

Applicable Conversion Price: _________________

Signature: _____________________
           [Name]

Address: _____________________

         _____________________


* This original Debenture must be received by the Company or its transfer agent
by the fifth business date following the Conversion Date.