STOCK PURCHASE AGREEMENT


          This Agreement is dated February 21, 2003 between Mainfield
Enterprises Inc. (the "Purchaser"), and Aphton Corporation (the "Company")
whereby the parties agree as follows:

          The Purchaser shall buy and the Company agrees to sell 500,000 shares
(the "Shares") of the Company's Common Stock at a price of $2.96 per share (the
average closing bid price of APHT common stock in the 5 business days prior to
the date of this agreement) for a total amount of $1,480,000. The Shares have
been registered on a Form S-3, File No. 333-92058, which registration statement
has been declared effective by the Securities and Exchange Commission. The
Shares are free of restrictive legends and are free of any resale restrictions.
The Company is delivering herewith a prospectus supplement on Form 424(b)(2)
regarding the sale of the Shares prior to funding.

          The Company has the requisite corporate power and authority to enter
into and to consummate the transactions contemplated by this Agreement and
otherwise to carry out its obligations hereunder. The execution and delivery of
this Agreement by the Company and the consummation by it of the transactions
contemplated hereunder have been duly authorized by all necessary action on the
part of the Company and no further consent or action is required by the Company,
its Board of Directors or its stockholders. This Agreement has been (or upon
delivery will be) duly executed by the Company and, when delivered in accordance
with the terms hereof, will constitute the valid and binding obligation of the
Company enforceable against the Company in accordance with its terms, except as
may be limited by bankruptcy, insolvency, reorganization, moratorium or similar
laws affecting creditors' right generally.

          The execution, delivery and performance of this Agreement by the
Company and the consummation by the Company of the transactions contemplated
hereby do not and will not (i) conflict with or violate any provision of the
Company's or any Subsidiary's certificate or articles of incorporation, bylaws
or other organizational or charter documents, or (ii) subject to obtaining the
Required Approvals (as defined below), conflict with, or constitute a default
(or an event that with notice or lapse of time or both would become a default)
under, or give to others any rights of termination, amendment, acceleration or
cancellation (with or without notice, lapse of time or both) of any agreement,
credit facility, debt or other instrument (evidencing a Company or Subsidiary
debt or otherwise) or other understanding to which the Company or any Subsidiary
is a party or by which any property or asset of the Company or any Subsidiary is
bound or affected, or (iii) result in a violation of any law, rule, regulation,
order, judgment, injunction, decree or other restriction of any court or
governmental authority to which the Company or a Subsidiary is subject
(including federal and state securities laws and regulations) and the rules and
regulations of any self-regulatory organization to which the Company or its
securities are subject, or by which any property or asset of the Company or a
Subsidiary is bound or affected; except in the case of each of clauses (ii) and
(iii), such as could not, individually or in the aggregate, have or result in a
material adverse effect on the business, properties, financial condition or
results of operations of the Company taken as a whole (a "Material Adverse
Effect").


<PAGE>


          Neither the Company nor any Subsidiary is required to obtain any
consent, waiver, authorization or order of, give any notice to, or make any
filing or registration with, any court or other federal, state, local or other
governmental authority or other Person in connection with the execution,
delivery and performance by the Company of this Agreement, other than (i) the
required filing of the Supplement, (ii) applicable Blue Sky filings, and (iii)
in all other cases where the failure to obtain such consent, waiver,
authorization or order, or to give such notice or make such filing or
registration could not have or result in, individually or in the aggregate, a
Material Adverse Effect (collectively, the "Required Approvals").

          The Shares and Warrants (collectively, the "Securities") are duly
authorized and, when issued and paid for in accordance with the terms hereof,
will be duly and validly issued, fully paid and nonassessable, free and clear of
all liens, encumbrances and rights of first refusal. The Company has reserved a
sufficient number of duly authorized shares of Common Stock to issue all of the
Securities. At the Closing, the Shares shall have been listed for trading on the
Nasdaq National Market (the "Trading Market").

          The Company's Registration Statement of Form S-3 (No.333-92058)
(the "Registration Statement") was declared effective by the Commission on
July 12, 2002. The Registration Statement is effective on the date hereof and
the Company has not received notice that the Commission has issued or intends to
issue a stop order with respect to the Registration Statement or that the
Commission otherwise has suspended or withdrawn the effectiveness of the
Registration Statement, either temporarily or permanently, or intends or has
threatened in writing to do so. The Registration Statement (including the
information or documents incorporated by reference therein), as of the time it
was declared effective, and any amendments or supplements thereto, each as of
the time of filing, did not contain any untrue statement of material fact or
omit to state any material fact required to be stated therein or necessary to
make the statements therein not misleading. The issuance of the Securities to
the Purchaser is registered by the Registration Statement.

          The Purchaser acknowledges that it has been afforded (i) the
opportunity to ask such questions as it has deemed necessary of, and to receive
answers from, representatives of the Company concerning the terms and conditions
of the offering of the Securities issued or issuable to it and the merits and
risks of investing in the Securities issued or issuable to it; (ii) access to
information about the Company and the Subsidiaries and their respective
financial condition, results of operations, business, properties, management and
prospects sufficient to enable it to evaluate its investment; and (iii) the
opportunity to obtain such additional information that the Company possesses or
can acquire without unreasonable effort or expense that is necessary to make an
informed investment decision with respect to the investment. Neither such
inquiries nor any other investigation conducted by or on behalf of such
Purchaser or its representatives or counsel shall modify, amend or affect such
Purchaser's right to rely on the truth, accuracy and completeness of all public
filings and the Company's representations and warranties contained in the
Transaction Documents.

          The Company confirms that neither it nor any other Person acting on
its behalf has provided any of the Purchasers or their agents or counsel with
any information that constitutes or might constitute material, non-public
information. The Company understands and confirms that the Purchasers will rely
on the foregoing representations in effecting transactions


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<PAGE>


in securities of the Company. All disclosure provided to the Purchasers
regarding the Company, its business and the transactions contemplated hereby,
including the Schedules to this Agreement, furnished by or on behalf of the
Company are true and correct and do not contain any untrue statement of a
material fact or omit to state any material fact necessary in order to make the
statements made therein, in light of the circumstances under which they were
made, not misleading.

          Except for listing fees to be paid by the Company to the Trading
Market and the placement fee payable to Wharton Capital Markets, LLC in
connection with the transactions contemplated by this Agreement, no fees or
commissions will be payable by the Company to any broker, financial advisor or
consultant, finder, placement agent, investment banker, bank or other Person
with respect to the transactions contemplated by this Agreement. The Purchaser
shall have no obligation with respect to any fees incurred by the Company or any
other Person (other than the Purchaser, if the Purchaser has agreed in writing
to pay such fees) or with respect to any claims made by or on behalf of other
Persons for fees of a type contemplated in this Section that may be due in
connection with the transactions contemplated by this Agreement. The Company
shall indemnify and hold harmless the Purchaser, its employees, officers,
directors, agents and partners, and its affiliates, from and against all claims,
losses, damages, costs (including the reasonable costs of preparation and
reasonable attorney's fees) and expenses suffered in respect of any such claimed
or existing fees incurred by the Company or any other Person (other than the
Purchaser, if the Purchaser has agreed in writing to pay such fees), as such
fees and expenses are incurred.

          The Company shall maintain a reserve from its duly authorized shares
of Common Stock for issuance pursuant to this Agreement in such amount as may be
required to fulfill its obligations in full under this Agreement and upon
exercise of the Warrant. In the event that at any time the then authorized
shares of Common Stock are insufficient for the Company to satisfy its
obligations in full under this Agreement and the Warrant, the Company shall
promptly take such actions as may be required to increase the number of
authorized shares. The Company shall take such steps as may be required to cause
and maintain the listing of the Shares on the Trading Market and such other
exchange, market or quotation facility on which the Common Stock is traded.

          Neither the Company nor the Purchaser shall issue any press release or
make any other public announcement relating to this Agreement unless (i) the
content thereof is mutually agreed to by the Company and the Purchaser, or (ii)
such party is advised by its counsel that such press release or public
announcement is required by law. The Company shall (i) within one Business Day
following the Closing Date, issue a press release mutually agreed to by the
Company and the Purchaser, disclosing the transactions contemplated hereby and
(ii) make such other filings and notices in the manner and time required by the
Commission.

          The Purchaser shall wire the purchase amount to the Company to the
account provided by the Company to the Purchaser.

          The Company shall cause its transfer agent to transmit the Shares
electronically to the Purchaser by crediting the account set forth below through
the Deposit Withdrawal Agent Commission system.


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<PAGE>


          Purchaser DWAC Instructions:

          Bear Stearns & Co., DTC # 352
          For the account of Mainfield Enterprises Inc.
          Account # 102-28262

          The Company and the Purchaser will sign a separate Warrant Agreement
as soon as possible, which agreement will grant 150,000 Warrants for a purchase
price of 12.5 cents per warrant, pursuant to which the Purchaser may purchase
from time to time, in whole or in part, up to an additional 150,000 shares from
the Company at a per share price of $2.96. The Warrants will have a five (5)
year life.

          The Warrant Agreement will include, without limitation, that the
Common Stock underlying the Warrants will be registered for resale as soon as
possible after the Closing Date. The Registration Statement will be filed within
30 days from the Closing Date. The Issuer shall respond to all SEC comments
within ten calendar days of receipt of said comments, and will use its best
efforts to cause the Registration Statement to become effective within 90 days
from the Closing Date.


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<PAGE>


                                  AGREED AND ACCEPTED

                                  Aphton Corp.



                                  By:     /s/ Philip Gevas
                                     -------------------------------
                                     Name:  Philip Gevas
                                     Title: Chairman & CEO



                                   PURCHASER:

                                   Mainfield Enterprises Inc.



                                   By:    /s/ Kenneth L. Henderson
                                      --------------------------------
                                      Name:  Kenneth L. Henderson
                                      Title:  Attorney-in-fact



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