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                                WARRANT AGREEMENT
                             FOR "CLASS B WARRANTS"

          AGREEMENT (this "Agreement") dated as of August 9, 1996, by and 
between XYTRONYX, INC., a Delaware corporation (the "Company") and AMERICAN 
STOCK TRANSFER & TRUST COMPANY, as warrant agent (the "Warrant Agent").

                               W I T N E S S E T H

          WHEREAS, in connection with the settlement of a class action 
lawsuit (the "Settlement") the Company will issue 309,734 Class B Warrants 
("Warrants"), each Warrant exercisable to purchase one share of common stock, 
par value $.02 per share of the Company ("Common Stock"); and 

          WHEREAS, the Company desires the Warrant Agent to act on behalf of 
the Company, and the Warrant Agent is willing to so act, in connection with 
the issuance, registration, transfer, exchange and redemption of the 
Warrants, the issuance of certificates representing the Warrants, the 
exercise of the Warrants, and the rights of the holders thereof;

          NOW THEREFORE, in consideration of the premises and the mutual 
agreements hereinafter set forth and for the purpose of defining the terms 
and provisions of the Warrants and the certificates representing the Warrants 
and the respective rights and obligations thereunder of the Company, the 
holders of certificates representing the Warrants and the Warrant Agent, the 
parties hereto agree as follows:

          SECTION 1.     DEFINITIONS.  As used herein, the following terms 
shall have the following meanings:

          (a)  "Common Stock" shall mean stock of the Company of any class, 
whether now or hereafter authorized, which has the right to participate in 
the distributions of earnings and assets of the Company without limit as to 
amount or percentage, which at the date hereof consists of 30,000,000 
authorized shares of Common Stock.

          (b)  "Corporate Office" shall mean the office of the Warrant Agent 
(or its successor) at which at any particular time its principal business 
shall be administered, which office is located at the date hereof at 40 Wall 
Street, New York, NY 10005.

          (c)  "Exercise Date" shall mean, as to any Warrant, the date on 
which the Warrant Agent shall have received both (a) the Warrant Certificate 
representing such Warrant, with the exercise form thereon duly executed by 
the Registered Holder thereof or his attorney duly authorized in writing, and 
(b) payment in cash, or by official bank or certified check made payable to 
the Company, of an amount in lawful money of the United States of America 
equal to the applicable Purchase Price.

          (d)  "Purchase Price" shall mean the purchase price to be paid upon 
exercise of each Warrant in accordance with the terms hereof, which price 
shall be $22.00 per share subject to adjustment from time to time pursuant to 
the provisions of Section 8 hereof, and subject to the Company's right to 
reduce the Purchase Price upon notice to all warrantholders.

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          (e)  "Registered Holder" shall mean the person in whose name any 
certificate representing Warrants shall be registered on the books maintained 
by the Warrant Agent pursuant to Section 6.

          (f)  "Transfer Agent" shall mean American Stock Transfer & Trust 
Company, as the Company's transfer agent, or its authorized successor, as 
such.

          (g)  "Warrant Expiration Date" shall mean 5:00 P.M. (New York time) 
on August 11, 2001; provided that if such date shall in the State of New York 
be a holiday or a day on which banks are authorized to close, then 5:00 P.M. 
(New York time) on the next following day which in the State of New York is 
not a holiday or a day on which banks are authorized to close.  Upon notice 
to all warrantholders the Company shall have the right to extend the Warrant 
Expiration Date.

SECTION 2.     WARRANTS AND ISSUANCE OF WARRANT CERTIFICATES.

          (a)  A Warrant shall initially entitle the Registered Holder of the 
Warrant Certificate representing such Warrant to purchase one share of Common 
Stock upon the exercise thereof, in accordance with the terms hereof, subject 
to modification and adjustment as provided in Section 8.

          (b)  From time to time, up to the Warrant Expiration Date, the 
Transfer Agent shall execute and deliver stock certificates in required whole 
number denominations representing up to an aggregate of 309,734 shares of 
Common Stock, subject to adjustment as described herein, upon the exercise of 
Warrants in accordance with this Agreement.

          (c)  From time to time, up to the Warrant Expiration Date, the 
Warrant Agent shall execute and deliver Warrant Certificates in required 
whole number denominations to the persons entitled thereto in connection with 
any transfer or exchange permitted under this Agreement; provided that no 
Warrant Certificates shall be issued except (i) those initially issued 
hereunder, (ii) those issued upon the exercise of fewer than all Warrants 
represented by any Warrant Certificate, to evidence any unexercised Warrants 
held by the exercising Registered Holder, (iii) those issued upon any 
transfer or exchange pursuant to Section 6; (iv) those issued in replacement 
of lost, stolen, destroyed or mutilated Warrant Certificates pursuant to 
Section 7; and (v) at the option of the Company, in such form as may be 
approved by the its Board of Directors, to reflect (a) any adjustment or 
change in the Purchase Price or the number of shares of Common Stock 
purchasable upon exercise of the Warrants, made pursuant to Section 8 hereof 
and (b) other modifications approved by Warrantholders in accordance with 
Section 16 hereof.

          SECTION 3. FORM AND EXECUTION OF WARRANT CERTIFICATES.  (a)  
The Warrant Certificates shall be substantially in the form annexed hereto as 
Exhibit A (the provisions of which are hereby incorporated herein) and may 
have such letters, numbers or other marks of identification or designation 
and such legends, summaries or endorsements printed, lithographed, engraved 
or typed thereon as the Company may deem appropriate and as are not 
inconsistent with the provisions of this Agreement, or as may be required to 
comply with any law or with any rule or regulation made pursuant thereto or 
with any rule or regulation of any stock exchange on which the Warrants may 
be listed, or to conform to usage.  The Warrant Certificates shall be

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dated the date of issuance thereof (whether upon initial issuance, transfer, 
exchange or in lieu of mutilated, lost, stolen, or destroyed Warrant 
Certificates) and issued in registered form. Warrants shall be numbered 
serially with the letters "WB."

          (b)  Warrant Certificates shall be executed on behalf of the 
Company by its Chairman of the Board, President or any Vice President and by 
its Secretary or an Assistant Secretary, by manual signatures or by facsimile 
signatures printed thereon, and shall have imprinted thereon a facsimile of 
the Company's seal.  In case any officer of the Company who shall have signed 
any of the Warrant Certificates shall cease to be such officer of the Company 
before the date of issuance of the Warrant Certificates and issue and 
delivery thereof, such Warrant Certificates may nevertheless be issued and 
delivered with the same force and effect as though the person who signed such 
Warrant Certificates had not ceased to be such officer of the Company.  After 
execution by the Company, Warrant Certificates shall be delivered by the 
Warrant Agent to the Registered Holder.

          SECTION 4.     EXERCISE.  Each Warrant may be exercised by the 
Registered Holder thereof at any time prior to the Warrant Expiration Date 
upon the terms and subject to the conditions, including without limitation, 
compliance with applicable securities laws, set forth herein and in the 
applicable Warrant Certificate.  A Warrant shall be deemed to have been 
exercised immediately prior to the close of business on the Exercise Date and 
the person entitled to receive the securities deliverable upon such exercise 
shall be treated for all purposes as the holder upon exercise thereof as of 
the close of business on the Exercise Date.  As soon as practicable on or 
after the Exercise Date, the Warrant Agent shall deposit the proceeds 
received from the exercise of a Warrant, and promptly after clearance of 
checks received in payment of the Purchase Price pursuant to such Warrants, 
cause to be issued and delivered by the Transfer Agent, to the person or 
persons entitled to receive the same, a certificate or certificates for the 
securities deliverable upon such exercise, (plus a certificate for any 
remaining unexercised Warrants of the Registered Holder).  Notwithstanding 
the foregoing, in the case of payment made in the form of a check drawn on an 
account of such investment banks and brokerage houses as the Company shall 
approve, certificates shall immediately be issued without any delay.  Upon 
the exercise of any Warrant and clearance of the funds received, the Warrant 
Agent shall promptly remit the payment received for the Warrant to the 
Company or as the Company may direct in writing.

          SECTION 5.     RESERVATION OF SHARES; LISTING; PAYMENT OF TAXES; ETC.
(a)  The Company covenants that it will at all times reserve and keep 
available out of its authorized Common Stock, solely for the purpose of issue 
upon exercise of Warrants, such number of shares of Common Stock as shall 
then be issuable upon the exercise of all outstanding Warrants. The Company 
covenants that all shares of Common Stock which shall be issuable upon 
exercise of the Warrants and payment of the Purchase Price shall, at the time 
of delivery, be duly and validly issued, fully paid, nonassessable and free 
from all taxes, liens and charges with respect to the issue thereof (other 
than those that arise as a result of the action or inaction of the Registered 
Holder).

          (b)  The Company will use reasonable efforts to obtain appropriate 
approvals or registrations under state "blue sky" securities laws with 
respect to the exercise of the Warrants; provided, however, that the Company 
shall not be obligated to file any general consent to service of process or 
qualify as a foreign corporation in any jurisdiction.  With respect to any 

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such securities laws, however, Warrants may not be exercised by, or shares of 
Common Stock issued to, any Registered Holder in any state in which such 
exercise would be unlawful.

          (c)  The Company shall pay all documentary, stamp or similar taxes 
and other governmental charges that may be imposed with respect to the 
issuance of Warrants, or the issuance, or delivery of any shares upon 
exercise of the Warrants; provided, however, that if the shares of Common 
Stock are to be delivered in a name other than the name of the Registered 
Holder of the Warrant Certificate representing any Warrant being exercised, 
then no such delivery shall be made unless the person requesting the same has 
paid to the Warrant Agent the amount of transfer taxes or charges incident 
thereto, if any.

          (d)  The Warrant Agent is hereby irrevocably authorized to 
requisition the Company's Transfer Agent from time to time for certificates 
representing shares of Common Stock required upon exercise of the Warrants, 
and the Company will authorize the Transfer Agent to comply with all such 
proper requisitions.

          SECTION 6.     EXCHANGE AND REGISTRATION OF TRANSFER. 

          Subject to the restrictions on transfer contained in the Warrant 
Certificates and the Subscription Agreements between the Company and the 
purchasers of Units:

          (a)  Warrant Certificates may be exchanged for other Warrant 
Certificates representing an equal aggregate number of Warrants of the same 
class or may be transferred in whole or in part; provided that no transfers, 
sales or other dispositions of the Warrants may be made except after the time 
periods and in the percentages set forth in Section 1.9 of the Subscription 
Agreements.  Warrant Certificates to be exchanged shall be surrendered to the 
Warrant Agent at its Corporate Office, and upon satisfaction of the terms and 
provisions hereof, the Company shall execute, and the Warrant Agent shall 
countersign, issue and deliver in exchange therefor the Warrant Certificate 
or Certificates which the Registered Holder making the exchange shall be 
entitled to receive.

          (b)  The Warrant Agent shall keep at its office books in which, 
subject to such reasonable regulations as it may prescribe, it shall register 
Warrant Certificates and the transfer thereof in accordance with its regular 
practice.  Upon due presentment for registration of transfer of any Warrant 
Certificate at its office, the Company shall execute and the Warrant Agent 
shall issue and deliver to the transferee or transferees a new Warrant 
Certificate or Certificates representing an equal aggregate number of 
Warrants.

          (c)  With respect to all Warrant Certificates presented for 
registration of transfer, or for exchange or exercise, the subscription form 
on the reverse thereof shall be duly endorsed, or be accompanied by a written 
instrument or instruments of transfer and subscription, in form satisfactory 
to the Company, duly executed by the Registered Holder or his 
attorney-in-fact duly authorized in writing.

          (d)  The Company may require payment by such holder of a sum 
sufficient to cover any tax or other governmental charge that may be imposed 
in connection therewith.


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          (e)  All Warrant Certificates surrendered for exercise or for 
exchange in case of mutilated Warrant Certificates shall be promptly canceled 
by the Warrant Agent and thereafter retained by the Warrant Agent until 
termination of this Agreement or resignation of the Warrant Agent, or, with 
the prior written consent of Paramount, disposed of or destroyed, at the 
direction of the Company.

          (f)  Prior to due presentment for registration of transfer thereof, 
the Company and the Warrant Agent may deem and treat the Registered Holder of 
any Warrant Certificate as the absolute owner thereof and of each Warrant 
represented thereby (notwithstanding any notations of ownership or writing 
thereon made by anyone other than a duly authorized officer of the Company or 
the Warrant Agent) for all purposes and shall not be affected by any notice 
to the contrary.

          SECTION 7.     LOSS OR MUTILATION.  Upon receipt by the Company and 
the Warrant Agent of evidence satisfactory to them of the ownership of and 
loss, theft, destruction or mutilation of any Warrant Certificate and (in 
case of loss, theft or destruction) of indemnity satisfactory to them, and 
(in the case of mutilation) upon surrender and cancellation thereof, the 
Company shall execute and the Warrant Agent shall (in the absence of notice 
to the Company and/or Warrant Agent that the Warrant Certificate has been 
acquired by a bonafide purchaser) countersign and deliver to the Registered 
Holder in lieu thereof a new Warrant Certificate of like tenor representing 
an equal aggregate number of Warrants.  Applicants for a substitute Warrant 
Certificate shall comply with such other reasonable regulations and pay such 
other reasonable charges as the Warrant Agent may prescribe.

          SECTION 8.     ADJUSTMENT OF PURCHASE PRICE AND NUMBER OF SHARES OF 
COMMON STOCK OR WARRANTS.  The Purchase Price in effect at any time and the 
number and kind of securities purchasable upon the exercise of the Warrants 
shall be subject to adjustment from time to time upon the happening of 
certain events as follows:

          (a)  In case the Company shall (i) declare a dividend or make a 
distribution on its outstanding shares of Common Stock in shares of Common 
Stock, (ii) subdivide or reclassify its outstanding shares of Common Stock 
into a greater number of shares, or (iii) combine or reclassify its 
outstanding shares of Common Stock into a smaller number of shares, the 
Exercise  Price in effect at the time of the record date for such dividend or 
distribution or the effective date of such subdivision, combination or 
reclassification shall be adjusted so that it shall equal the price 
determined by multiplying the Purchase Price by a fraction, the denominator 
of which shall  be the number of shares of Common Stock outstanding after 
giving effect to such action, and the numerator of which shall be the number 
of shares of Common Stock outstanding immediately prior to such action.  Such 
adjustment shall be successively whenever any event listed above shall occur.

          (b)  Whenever the Purchase Price payable upon exercise of each 
Warrant is adjusted pursuant to Subsection (a) above, the number of Shares 
purchasable upon exercise of this Warrant shall simultaneously be adjusted by 
multiplying the number of Shares initially issuable upon exercise of this 
Warrant by the Purchase Price in effect on the date hereof and dividing the 
product so obtained by the Purchase Price, as adjusted.

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          (c)  No adjustment in the Purchase Price shall be required unless 
such adjustment would require an increase or decrease of at least five cents 
(0.05) in such price; provided, however, that any adjustments which by reason 
of this Subsection (c) are not required to be made shall be carried forward 
and taken into account in any subsequent adjustment required to be made 
hereunder.  All calculations under this Section 8 shall be made to the 
nearest cent or to the nearest one-hundredth of a share, as the case may be.  
Anything in this Section 8 to the contrary notwithstanding, the Company shall 
be entitled, but shall not be required, to make such changes in the Purchase 
Price, in addition to those required by this Section 8 as it shall determine, 
in its sole discretion, to be advisable in order that any dividend or 
distribution in shares of Common Stock or any subdivision, reclassification 
or combination of Common Stock, hereafter made by the Company shall not 
result in any Federal Income tax liability to the Registered Holder of Common 
Stock or securities convertible into Common Stock (including Warrants).

          (d)  Whenever the Purchase Price is adjusted, as herein provided, 
the Company shall promptly but no later than 20 days after any request for 
such an adjustment by the Registered Holder, cause a notice setting forth the 
adjusted Purchase Price and adjusted number of Shares issuable upon exercise 
of each Warrant, and, if requested, information describing the transactions 
giving rise to such adjustments, to be mailed to the Registered Holder at his 
last address appearing in the warrant register of the Warrant Agent, and 
shall cause a certified copy thereof  to be mailed to its Warrant Agent.  The 
Company may retain a firm of independent certified public accountants 
selected by the Board of Directors (who may be the regular accountants 
employed by the Company) to make any computations required by this Section 8, 
and a certificate signed by such firm shall be conclusive evidence of the 
correctness of such adjustment.

          (e)  In the event that at any time, as a result of an adjustment 
made pursuant to Subsection (a) above, the Registered Holder of this Warrant 
thereafter shall become entitled to receive any shares of the Company, other 
than Common Stock, thereafter the number of such other shares so receivable 
upon exercise of this Warrant shall be subject to adjustment from time to 
time in a manner and on terms as nearly equivalent as practicable to the 
provisions with respect to the Common Stock contained in Subsection (a) 
above.

          (f)  Irrespective of any adjustments in the Purchase Price or the 
number or kind of shares purchasable upon exercise of this Warrant, Warrants 
theretofore or thereafter issued may continue to express the same price and 
number and kind of shares as are stated in the similar Warrants initially 
issuable pursuant to this Agreement.

          SECTION 9.     RECLASSIFICATION, REORGANIZATION OR MERGER.  In case 
of any reclassification, capital reorganization or other change of 
outstanding shares of Common Stock of the Company, or in case of any 
consolidation  or merger of the Company with or into another corporation 
(other than a merger with a subsidiary in which merger the Company is the 
continuing corporation and which does not result in any reclassification, 
capital reorganization or other change of outstanding shares of Common Stock 
of the class issuable upon exercise of this Warrant) or in case of any sale, 
lease or conveyance to another corporation of the property of the Company as 
an entirety, the Company shall, as a condition precedent to such transaction, 
cause effective provisions to be made so that the holder shall have the right 
thereafter by exercising this Warrant at any time prior to the expiration of 
the Warrant, to purchase the kind and amount

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of shares of stock and other securities and property receivable upon such 
reclassification, capital reorganization and other change, consolidation, 
merger, sale or conveyance by a holder of the number of shares of Common 
Stock which might have been purchased upon exercise of this Warrant 
immediately prior to such reclassification, change, consolidation, merger, 
sale or conveyance. Any such provision shall include provision for 
adjustments which shall be as nearly equivalent as may be practicable to the 
adjustments provided for in this Warrant.   The foregoing provisions of this 
Section 9 shall similarly apply to successive reclassifications, capital 
reorganizations and changes of shares of Common Stock and to successive 
consolidations, mergers, sales or conveyances. In the event that in 
connection with any such capital reorganization or reclassification, 
consolidation, merger, sale or conveyance, additional shares of Common Stock 
shall be issued in exchange, conversion, substitution or payment, in whole or 
in part, for a security of the Company other that Common Stock, any such 
issue shall be treated as an issue of Common Stock covered by the provisions 
of Subsection (a) of Section (8) hereof.

          SECTION 10.    FRACTIONAL WARRANTS AND FRACTIONAL SHARES.

          (a)  If the number of shares of Common Stock purchasable upon the 
exercise of each Warrant is adjusted pursuant to Section 8 hereof, the 
Company shall nevertheless not be required to issue fractions of shares, upon 
exercise of the Warrants or otherwise, or to distribute certificates that 
evidence fractional shares.  With respect to any fraction of a share called 
for upon any exercise hereof, the Company shall pay to the Holder an amount 
in cash equal to such fraction multiplied by the current market value of such 
fractional share, determined as follows:

          (1)  If the Common Stock is listed on a national securities 
               exchange or admitted to unlisted trading privileges on such 
               exchange or listed for trading on the Nasdaq National Market 
               System ("NMS"), the current market value shall be the last 
               reported sale price of the Common Stock on such exchange on 
               the last business day prior to the date of exercise of this 
               Warrant or if no such sale is made on such day or no closing 
               sale price is quoted, the average of the closing bid and asked 
               prices for such day on such exchange or system; or

          (2)  If the Common Stock is listed in the over-the-counter market 
               (other than on NMS) or admitted to unlisted trading 
               privileges, the current market value shall be the mean of the 
               last reported bid and asked prices reported by the National 
               Quotation Bureau, Inc. on the last business day prior to the 
               date of the exercise of this Warrant; or

          (3)  If the Common Stock is not so listed or admitted to unlisted 
               trading privileges and bid and asked prices are not so 
               reported, the current market value shall be prescribed by the 
               Board of Directors of the Company.

          SECTION 11.    WARRANT HOLDERS NOT DEEMED STOCKHOLDERS.  No holder 
of Warrants shall, as such, be entitled to vote or to receive dividends or be 
deemed the holder of Common Stock that may at any time be issuable upon 
exercise of such Warrants for any purpose whatsoever, nor shall anything 
contained herein be construed to confer upon the holder of Warrants, as such, 
any of the rights of a stockholder of the Company or any right to vote for 
the election of directors or upon any matter submitted to stockholders at any 
meeting thereof, or to give or withhold consent to any corporate action 
(whether upon any recapitalization, issue or

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reclassification of stock, change of par value or change of stock to no par 
value, consolidation, merger or conveyance or otherwise), or to receive 
notice of meetings, or to receive dividends or subscription rights, until 
such Holder shall have exercised such Warrants and been issued shares of 
Common Stock in accordance with the provisions hereof.

          SECTION 12.    RIGHTS OF ACTION.  All rights of action with respect 
to this Agreement are vested in the respective Registered Holders of the 
Warrants, and any Registered Holder of a Warrant, without consent of the 
Warrant Agent or of the holder of any other Warrant, may, on his own behalf 
and for his own benefit, enforce against the Company his right to exercise 
his Warrants for the purchase of shares of Common Stock in the manner 
provided in the Warrant Certificate and this Agreement.

          SECTION 13.    AGREEMENT OF WARRANT HOLDERS.  Every holder of a 
Warrant, by his acceptance thereof, consents and agrees with the Company, the 
Warrant Agent and every other holder of a Warrant that:

          (a)  The Warrants are transferable only on the registry books of 
the Warrant Agent by the Registered Holder thereof in person or by his 
attorney duly authorized in writing and only if the Warrant Certificates 
representing such Warrants are surrendered at the office of the Warrant 
Agent, duly endorsed or accompanied by a proper instrument of transfer 
satisfactory to the Warrant Agent and the Company in their sole discretion, 
together with payment of any applicable transfer taxes; and

          (b)  The Company may deem and treat the person in whose name the 
Warrant Certificate is registered as the holder and as the absolute, true and 
lawful owner of the Warrants represented thereby for all purposes, and the 
Company shall not be affected by any notice or knowledge to the contrary, 
except as otherwise expressly provided in Section 7 hereof.

          SECTION 14.    CANCELLATION OF WARRANT CERTIFICATES.  If the 
Company shall purchase or acquire any Warrant or Warrants, the Warrant 
Certificate or Warrant Certificates evidencing the same shall thereupon be 
canceled by it and retired.  The Warrant Agent shall also cancel Common Stock 
following exercise of any or all of the Warrants represented thereby or 
delivered to it for transfer, split up, combination or exchange.

          SECTION 15.    CONCERNING THE WARRANT AGENT.  The Warrant Agent 
acts hereunder as agent and in a ministerial capacity for the Company, and 
its duties shall be determined solely by the provisions hereof.  The Warrant 
Agent shall not, by issuing and delivering Warrant Certificates or by any 
other act hereunder be deemed to make any representations as to the validity, 
value or authorization of the Warrant Certificates or the Warrants 
represented thereby or of any securities or other property delivered upon 
exercise of any Warrant or whether any stock issued upon exercise of any 
Warrant is fully paid and nonassessable.

          The Warrant Agent shall account promptly to the Company with 
respect to Warrants exercised and concurrently pay the Company, as provided 
in Section 4, all moneys received by the Warrant Agent upon the exercise of 
such Warrants.  The Warrant Agent shall, upon request of the Company from 
time to time, deliver to the Company such complete reports of registered 
ownership of the Warrants and such complete records of transactions with 
respect

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to the Warrants and the shares of Common Stock as the Company may request.  
The Warrant Agent shall also make available to the Company and Paramount for 
inspection by their agents or employees, from time to time as either of them 
may request, such original books of accounts and record (including original 
Warrant Certificates surrendered to the Warrant Agent upon exercise of 
Warrants) as may be maintained by the Warrant Agent in connection with the 
issuance and exercise of Warrants hereunder, such inspections to occur at the 
Warrant Agent's office as specified in Section 17, during normal business 
hours.

          The Warrant Agent shall not at any time be under any duty or 
responsibility to any holder of Warrant Certificates to make or cause to be 
made any adjustment of the Purchase Price provided in this Agreement, or to 
determine whether any fact exists which may require any such adjustments, or 
with respect to the nature or extent of any such adjustment, when made, or 
with respect to the method employed in making the same.  It shall not (i) be 
liable for any recital or statement of facts contained herein or for any 
action taken, suffered or omitted by it in reliance on any Warrant 
Certificate or other document or instrument believed by it in good faith to 
be genuine and to have been signed or presented by the proper party or 
parties, (ii) be responsible for any failure on the part of the Company to 
comply with any of its covenants and obligations contained in this Agreement 
or in any Warrant Certificate, or (iii) be liable for any act or omission in 
connection with this Agreement except for its own negligence or willful 
misconduct.

          The Warrant Agent may at any time consult with counsel satisfactory 
to it (who may be counsel for the Company) and shall incur no liability or 
responsibility for any action taken, suffered or omitted by it in good faith 
in accordance with the written opinion or advice of such counsel.

          Any notice, statement, instruction, request, direction, order or 
demand of the Company shall be sufficiently evidenced by an instrument signed 
by the Chairman of the Board, President, any Vice President, its Secretary, 
or Assistant Secretary, (unless other evidence in respect thereof is herein 
specifically prescribed).  The Warrant Agent shall not be liable for any 
action taken, suffered or omitted by it in accordance with such notice, 
statement, instruction, request, direction, order or demand believed by it to 
be genuine.

          The Company agrees to pay the Warrant Agent reasonable compensation 
for its services hereunder and to reimburse it for its reasonable expenses 
hereunder; it further agrees to indemnify the Warrant Agent and save it 
harmless against any and all losses, expenses and liabilities, including 
judgments, costs and counsel fees, for anything done or omitted by the 
Warrant Agent in the execution of its duties and powers hereunder except 
losses, expenses and liabilities arising as a result of the Warrant Agent's 
negligence or willful misconduct.

          The Warrant Agent may resign its duties and be discharged from all
further duties and liabilities hereunder (except liabilities arising as a result
of the Warrant Agent's own negligence or willful misconduct), after giving 30
days' prior written notice to the Company.  At least 15 days prior to the date
such resignation is to become effective, the Warrant Agent shall cause a copy of
such notice of resignation to be mailed to the Registered Holder of each Warrant
Certificate at the Company's expense.  Upon such resignation, or any inability
of the Warrant Agent to act as such hereunder, the Company shall appoint a new
warrant agent in writing.  If the Company shall fail to make such appointment
within a period of 15 days after it has been

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notified in writing of such resignation by the resigning Warrant Agent, then 
the Registered Holder of any Warrant Certificate may apply to any court of 
competent jurisdiction for the appointment of a new warrant agent.  Any new 
warrant agent, whether appointed by the Company or by such a court, shall be 
a bank or trust company having a capital and surplus, as shown by its last 
published report to its stockholders, of not less than $10,000,000 or a stock 
transfer company.  After acceptance in writing of such appointment by the new 
warrant agent is received by the Company, such new warrant agent shall be 
vested with the same powers, rights, duties and responsibilities as if it had 
been originally named herein as the Warrant Agent, without any further 
assurance, conveyance, act or deed; but if for any reason it shall be 
necessary or expedient to execute and deliver any further assurance, 
conveyance, act or deed, the same shall be done at the expense of the Company 
and shall be legally and validly executed and delivered by the resigning 
Warrant Agent.  Not later than the effective date of any such appointment the 
Company shall file notice thereof with the resigning Warrant Agent and shall 
forthwith cause a copy of such notice to be mailed to the Registered Holder 
of each Warrant Certificate.

          Any corporation into which the Warrant Agent or any new warrant 
agent may be converted or merged or any corporation resulting from any 
consolidation to which the Warrant Agent or any new warrant agent shall be a 
party or any corporation succeeding to the trust business of the Warrant 
Agent shall be a successor warrant agent under this Agreement without any 
further act, provided that such corporation is eligible for appointment as 
successor to the Warrant Agent under the provisions of the preceding 
paragraph.  Any such successor warrant agent shall promptly cause notice of 
its succession as warrant agent to be mailed to the Company and to the 
Registered Holder of each Warrant Certificate.

          The Warrant Agent, its subsidiaries and affiliates, and any of its 
or their officers or directors, may buy and hold or sell Warrants or other 
securities of the Company and otherwise deal with the Company in the same 
manner and to the same extent and with like effects as though it were not 
Warrant Agent.  Nothing herein shall preclude the Warrant Agent from acting 
in any other capacity for the Company or for any other legal entity.

          SECTION 16.    MODIFICATION OF AGREEMENT.  The parties hereto may 
by supplemental agreement make any changes or corrections in this Agreement 
(i) that it shall deem appropriate to cure any ambiguity or to correct any 
defective or inconsistent provision or manifest mistake or error herein 
contained; (ii) that it may deem necessary or desirable and which shall not 
adversely affect the interests of the holders of Warrant Certificates; 
PROVIDED, HOWEVER, that this Agreement shall not otherwise be modified, 
supplemented or altered in any respect except with the consent in writing of 
the Registered Holders of Warrant Certificates representing not less than 50% 
of the Warrants then outstanding; and PROVIDED, FURTHER, that no change in 
the number or nature of the securities purchasable upon the exercise of any 
Warrant, or the Purchase Price therefor, or the acceleration of the Warrant 
Expiration Date, shall be made without the consent in writing of the 
Registered Holder of the Warrant Certificate representing such Warrant, other 
than such changes as are specifically prescribed by this Agreement as 
originally executed.

          SECTION 17.    NOTICES.  All notices, requests, consents and other
communications hereunder shall be in writing and shall be deemed to have been
made when delivered or mailed first class registered or certified mail, postage
prepaid as follows: if to the

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<PAGE>

Registered Holder of a Warrant Certificate, at the address of such holder as 
shown on the registry books maintained by the Warrant Agent; if to the 
Company, at 6555 Nancy Ridge Drive, Suite 200, San Diego, California 92121, 
Attention:  Dale A. Sander, and if to the Warrant Agent, at 40 Wall Street, 
New York, NY 10005, Attention               .

          SECTION 18.    GOVERNING LAW.  This Agreement shall be governed by 
and construed in accordance with the laws of the State of New York, without 
reference to its  principles of conflict of laws.

          SECTION 19.    BINDING EFFECT.  This Agreement shall be binding 
upon and inure to the benefit of the Company and the Warrant Agent (and their 
respective successors and assigns) and the holders from time to time of 
Warrant Certificates.  Nothing in this Agreement is intended or shall be 
construed to confer upon any other person any right, remedy or claim, in 
equity or at law, or to impose upon any other person any duty, liability or 
obligation.

          SECTION 20.    TERMINATION.  This Agreement shall terminate on the 
earlier to occur of (i)  the close of business on the Expiration Date of all 
the Warrants; (ii) the date upon which all Warrants have been exercised.

          SECTION 21.    COUNTERPARTS.  This Agreement may be executed in 
several counterparts, which taken together shall constitute a single document.

          IN WITNESS WHEREOF, the parties hereto have caused this Agreement 
to be duly executed as of the date first above written.

                              XYTRONYX, INC.



                              By: /s/   Dale A. Sander              
                                  ----------------------------------
                                Dale A. Sander
                                Chief Financial Officer


                              AMERICAN STOCK TRANSFER 
                              & TRUST COMPANY


                              By: /s/  Herbert J. Lemmer             
                                  ----------------------------------
                                Name:  Herbert J. Lemmer
                                Title:    Vice-President    


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