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                                    XYTRONYX, INC.

                     STOCK OPTION PLAN FOR NON-EMPLOYEE DIRECTORS

    1.  Purpose.  The purpose of the Xytronyx, Inc. Stock Option Plan for
Non-Employee directors (the "Plan") is to promote the interests of Xytronyx,
Inc. (the "Company") and its stockholders by strengthening the Company's ability
to attract and retain the services of experienced and knowledgeable non-employee
directors and by encouraging such directors to acquire an increased proprietary
interest in the Company.

    2.  Shares Subject to the Plan.  Subject to adjustment as provided in 
Article 7, the total number of shares of common stock (the "Common Stock") of 
the Company for which options may be granted under the Plan shall be 
3,000,000 shares of Common Stock (the "Shares").  The Shares shall be shares 
currently authorized but unissued or currently held or subsequently acquired 
by the Company as treasury shares, including shares purchased in the open 
market or in private transactions.  If any option granted under the Plan 
expires or terminates for any reason without having been exercised in full, 
the Shares subject to, but not delivered under, such options may become 
available for that grant of other options under the Plan.  No shares 
delivered to the Company in full or partial payment of an option exercise 
price payable pursuant to Section 6.3 shall become available for the grant of 
other options under the Plan.

    3.  Administration of the Plan.  The Plan shall be administered by the 
Compensation Committee of the Company's Board of Directors (the "Committee"), 
subject to Articles 10 and 11.  Subject to the terms of the Plan, the 
Committee shall have the power to construe the provisions of the Plan, to 
determine all questions arising thereunder, and to adopt and amend such rules 
and regulations for administering the Plan as the Committee deems desirable.

    4.  Participation in the Plan.  Each member of the Company's Board of 
Directors (a "Director") who is not otherwise an employee of the Company or 
any subsidiary of the Company (an "Eligible Director") shall be eligible to 
participate in the Plan.

    5.  Nonstatutory Stock Options.  All options granted under the Plan shall 
be nonstatutory options not intended to qualify under Section 422 of the 
Internal Revenue Code of 1986, as amended.

    6.  Option Terms.  Each option granted to an Eligible Director under the
Plan and the issuance of Shares thereunder shall be subject to the following
terms:

    6.1  Option Agreements.  Each option granted under the Plan shall be 
evidenced by an option agreement (an "Agreement") duly executed on behalf of 
the Company and by the Eligible Director to whom such option is granted and 
dated as of the applicable date of grant.  Each Agreement shall be signed on 
behalf of the Company by an officer or officers delegated such authority by 
the Committee using either manual or 



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facsimile signature.  Each Agreement shall comply with and be subject to the 
terms and conditions of the Plan.  Any Agreement may contain such other 
terms, provisions and conditions not inconsistent with the Plan as may be 
determined by the Committee.

    6.2.  Option Grant Size and Grant dates.

         6.2.1  Initial Grants.  An option to purchase 50,000 Shares as
adjusted pursuant to Article 7 (an "Initial Grant") shall be granted to:

              a.  each Director who is an Eligible Director on the Effective
Date (as hereinafter defined), and

              b.  each other Eligible Director immediately following the Annual
Meeting at which such Director is first elected to be a Director or at the close
of business on the day upon which such Eligible Director is first appointed by
the Board to be a Director, whichever first occurs; provided, that if an
Eligible Director who previously received an Initial Grant terminates service as
a Director and is subsequently elected or appointed to the Board, such Director
shall not be eligible to receive a second Initial Grant, but shall be eligible
to receive only Annual Grants as provided in Section 6.2.2.

         6.2.2  Annual Grants.  An option to purchase 10,000 Shares as adjusted
pursuant to Article 7 (an "Annual Grant"), shall be granted automatically each
year, immediately following the Annual Meeting, to each Director who is an
Eligible Director at such time.

    6.3  Option Exercise Price. Each Agreement shall state the exercise price
per share of the shares of Common Stock to which it relates.  The exercise price
per share of Common Stock subject to an option shall not be less than 100% of
the fair market value ("Fair Market Value") per share of such Common Stock at
the close of business on the day of the grant of the option.  For purposes of
this Plan, Fair Market Value on any date shall be the mean of the high and low
sales prices per share of Common Stock on the relevant date as reported on the
stock exchange or market on which the Common Stock is primarily traded, or if no
sale is made on such date, then the Fair Market Value is the weighted average of
the mean of the high and low sales prices per share of Common Stock on the next
preceding day and the next succeeding day on which such sales were made, as
reported on the stock exchange or market on which the Common Stock is primarily
traded.

    6.4  Exercisability.  Subject to Section 6.7, an option shall incrementally
vest and become exercisable in twelve equal portions (subject to necessary
rounding) for each quarter of each year of the three year period beginning the
day on which such option was granted, if the optionee has continued to serve as
a Director until that day.

    6.5  Time and Manner of Option Exercise.  Any vested and exercisable option
is exercisable in whole or in part at any time or from time to time during the
term of the option period by giving written 



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notice, signed by the person exercising the option, to the Company stating 
the number of Shares with respect to which the option is being exercised, 
accompanied by payment in full of the option exercise price for the number of 
Shares to be purchased and by the payment or making provision satisfactory to 
the Company for the payment of any taxes which the Company is obligated to 
collect with respect to the issue or transfer of the Shares upon such 
exercise.  The date both such notice and payment are received by the office 
of the Secretary of the Company shall be the date of exercise for the stock 
option as to such number of Shares.  No option may at any time be exercised 
with respect to a fractional Share.

    6.6  Payment of Exercise Price.  Payment of the option exercise price may
be in cash or payment may be in whole or part by

              a.  transfer to the Company of shares of Common Stock having a
Fair Market Value equal to the option exercise price at the time of such
exercise, or

              b.  delivery of instructions to the Company to withhold Shares,
that would otherwise be issued on such exercise of the option, having a Fair
Market Value at the time of such exercise equal to the total option exercise
price of the options being exercised.

    If the Fair Market Value of the number of whole shares transferred or the
number of whole option Shares surrendered is less than the total exercise price
of the option being exercised, the shortfall must be made up in cash.

    6.7  Terms of Options.  Each option shall expire ten years from its date of
grant, but shall be subject to earlier termination as follows:

              a.  In the event of the termination of an optionee's services as
a Director by reason of voluntary mid-term retirement, declining to stand for
re-election, becoming a full time employee of the Company or a subsidiary of the
Company or becoming disabled, all options granted pursuant to this Plan but
unexercisable pursuant to Section 6.4 shall automatically expire and shall not
be exercisable and all options exercisable pursuant to Section 6.4 but
unexercised shall continue to be exercisable until the stated expiration date of
such options.

              b.  In the event of the death of an optionee or total disability
while the optionee is a Director, the then outstanding options of such optionee
that have vested pursuant to Section 6.4 shall be exercisable for one year from
the date of the death of the optionee or until the stated grant expiration date,
whichever is earlier, by his/her successors in interest, in accordance with the
paragraph below.  However, all options which have been granted, but have not
become exercisable pursuant to Section 6.4, shall automatically expire.

              c.  In the event of the termination of an optionee's service as a
Director by the Board of Directors for cause or the failure 

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of such Director to be re-elected (other than for the reasons set forth in 
Section 6.7 (a) or (b)), the Committee in its sole discretion can cancel the 
then-outstanding options of such optionee, including those options which are 
exercisable and such options shall automatically expire and become 
non-exercisable on the effective date of such termination.

    Exercise of a deceased optionee's options that are still exercisable shall
be by the estate of such optionee or by a person or persons whom the optionee
has designated in writing filed with the Company, or, if no such designation has
been made, by the person or persons to whom the optionee's rights have passed by
will or the laws of descent and distribution.

    6.8  Transferability.  The right of any optionee to exercise an option
granted under the Plan shall, during the lifetime of such optionee, be
exercisable only by the optionee and shall not be assignable or transferable by
such optionee other than by will or the laws of descent and distribution.

    6.9  Limitation of Rights.

         6.9.1  Limitation as to Shares.  Neither the recipient of an option
under the Plan nor an optionee's successor or successors in interest shall have
any rights as a stockholder of the Company with respect to any Shares subject to
an option granted to such person until the date of issuance of a stock
certificate for such Shares.

         6.9.2  Limitation as to Directorship.  Neither the Plan, nor the
granting of an option, nor any other action taken pursuant to the Plan shall
constitute or be evidence of any agreement or understanding, express or implied,
that an Eligible Director has a right to continue as a Director for any period
of time or at any particular rate of compensation.

    6.10  Regulatory Approval and Compliance.  The Company shall not be
required to issue any certificate or certificates for Shares upon the exercise
of an option granted under the Plan or to record as a holder of record of Shares
the name of the individual exercising an option under the Plan, without
obtaining to the complete satisfaction of the Committee the approval of all
regulatory bodies deemed necessary by the Committee and without complying, to
the Committee's complete satisfaction, with all rules and regulations under
federal, state, or local law deemed applicable by the Committee.

    7.  Capital Adjustments.  The aggregate number and class of Shares subject
to and authorized by the Plan, the number of class of Shares with respect to
which an option may be granted to an Eligible Director under the Plan as
provided in Article 6, the number and class of Shares subject to each
outstanding option, and the exercise price per share specified in each such
option shall be proportionately adjusted for any increase or decrease in the
number of issued shares of Common Stock resulting from a split-up or
consolidation of shares or any like capital 

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adjustment or the payment of any stock dividend, or other increase or 
decrease in the number of such Shares effected without receipt of 
consideration by the Company.

    8.  Effectiveness of the Plan.  The Plan shall be effective as of December
19, 1996 (the "Effective Date"), subject to the approval by the Company's
stockholders.  All options issued prior to the date of the approval of the Plan
by the Company's stockholders shall be issued subject to such approval.  The
Plan shall continue in effect until it is terminated by action of the Board or
the Company's stockholders, but such termination shall not affect the terms of
any then outstanding options.

    9.  Termination and Amendment of the Plan.  The Board may amend, terminate
or suspend the Plan at any time, in its sole and absolute discretion; provided,
however, that if required to qualify the Plan under Rule 16b-3 promulgated under
Section 16, of the Securities Exchange Act of 1934, as amended ("Rule 16b-3"),
no amendment shall be made more than once every six months that would change the
amount, price or timing of the Initial and Annual Grants, other than to comport
with changes in the Internal Revenue Code of 1986, as amended, or the rules and
regulations promulgated thereunder; and provided, further, that if required to
qualify the Plan under the Rule 16b-3, no amendment that would

              a.  materially increase the number of Shares that may be issued
under the Plan,

              b.  materially modify the requirements as to eligibility for
participation in the Plan, or

              c.  otherwise materially increase the benefits accruing to
participants under the Plan shall be made without the approval of the Company's
stockholders.

    10.  Compliance with Rule 16b-3.  Other provisions of the Plan
notwithstanding, neither the Committee nor any other person (other than an
Eligible Director acting in conformity with the terms of the Plan) shall have
any discretionary authority to make determinations regarding the Plan required
by Rule 16b-3 to be afforded exclusively to "disinterested persons" as defined
thereunder.