<PAGE>

                                                                  EXHIBIT 10.28


THE SECURITIES EVIDENCED BY THIS WARRANT CERTIFICATE HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), AND MAY NOT
BE SOLD, TRANSFERRED, OR ASSIGNED UNLESS THERE IS AN EFFECTIVE REGISTRATION
STATEMENT UNDER SUCH ACT COVERING SUCH SECURITIES, THE SALE IS MADE IN
ACCORDANCE WITH RULE 144 UNDER THE SECURITIES ACT, OR THE COMPANY RECEIVES AN
OPINION OF COUNSEL FOR THE HOLDER OF SUCH SECURITIES REASONABLY SATISFACTORY TO
THE COMPANY STATING THAT SUCH SALE, TRANSFER, ASSIGNMENT OR HYPOTHECATION IS
EXEMPT FROM THE REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENTS OF SUCH ACT.

No.:  ____


                                     FORM OF
                                     WARRANT
                            TO PURCHASE COMMON STOCK
                                       OF
                             LYNX THERAPEUTICS, INC.

                           (void after April __, 2007)


        1. Issuance of Warrant. FOR VALUE RECEIVED, on and after the date of
issuance of this Warrant, and subject to the terms and conditions herein set
forth, [o] or its registered assigns (the "Holder") is entitled to purchase from
Lynx Therapeutics, Inc., a Delaware corporation (the "Company"), at any time
before 5:00 p.m. Pacific time on April __, 2007 (the "Termination Date"), at a
price per share equal to the Warrant Price (as defined below and subject to
adjustment as described below), the Warrant Stock (as defined below and subject
to adjustment as described below) upon exercise of this warrant (this "Warrant")
pursuant to Section 6 hereof.

        2. Definitions. As used in this Warrant, the following terms have the
definitions ascribed to them below:

           (a) "Business Day" means any day other than a Saturday, Sunday or
other day on which the national or state banks located in the Commonwealth of
Virginia are authorized to be closed.

           (b) "Common Stock" means the common stock, par value $0.01 per share,
of the Company.

           (c) "Purchase Agreement" means the Securities Purchase Agreement
dated as of April__, 2002 by and among the Company, the Holder and the other
investors named therein.

           (d) "Warrant Price" means $1.94 per share, subject to adjustment as
described in Section 3 below.


<PAGE>


           (e) "Warrant Stock" means the shares of Common Stock (or other
securities) purchasable upon exercise of this Warrant or issuable upon
conversion of this Warrant. The total number of shares to be issued upon the
exercise of this Warrant shall be _______, subject to adjustment as described in
Section 3 below.

        3. Adjustments and Notices. The Warrant Price and the number of shares
of Warrant Stock shall be subject to adjustment from time to time in accordance
with this Section 3.

           (a) Subdivision, Stock Dividends or Combinations. In case the Company
shall at any time subdivide the outstanding shares of Common Stock or shall
issue a stock dividend with respect to the Common Stock, the Warrant Price in
effect immediately prior to such subdivision or the issuance of such dividend
shall be proportionately decreased, and in case the Company shall at any time
combine the outstanding shares of the Common Stock, the Warrant Price in effect
immediately prior to such combination shall be proportionately increased, in
each case effective at the close of business on the date of such subdivision,
dividend or combination, as the case may be.

           (b) Reclassification, Exchange, Substitution, In-Kind Distribution.
Upon any reclassifications, exchange, substitution or other event that results
in a change of the number and/or class of the securities issuable upon exercise
or conversion of this Warrant or upon the payment of a dividend in securities or
property other than shares of Common Stock, the Holder shall be entitled to
receive, upon exercise or conversion of this Warrant, the number and kind of
securities and property that the Holder would have received if this Warrant had
been exercised or converted immediately before the record date for such
reclassification, exchange, substitution, or other event or immediately prior to
the record date for such dividend. The Company or its successor shall promptly
issue to the Holder a new warrant for such new securities or other property. The
new warrant shall provide for adjustments which shall be as nearly equivalent as
may be practicable to the adjustments provided for in this Section 3 including,
without limitation, adjustments to the Warrant Price and to the number of
securities or property issuable upon exercise or conversion of the new warrant.
The provisions of this Section 3(b) shall similarly apply to successive
reclassifications, exchanges, substitutions, or other events and successive
dividends.

           (c) Reorganization, Merger etc. In case of any (i) merger or
consolidation of the Company into or with another corporation where the Company
is not the surviving corporation, (ii) sale, transfer or lease (but not
including a transfer or lease by pledge or mortgage to a bona fide lender) of
all or substantially all of the assets of the Company or (iii) sale by the
Company's shareholders of 50% or more of the Company's outstanding securities in
one or more related transactions, the Company, or such successor or purchasing
corporation, as the case may be, shall, as a condition to closing any such
reorganization, merger or sale, duly execute and deliver to the Holder hereof a
new warrant so that the Holder shall have the right to receive, at a total
purchase price not to exceed that payable upon the exercise or conversion of the
unexercised or unconverted portion of this Warrant, and in lieu of the shares of
the Common Stock theretofore issuable upon exercise or conversion of this
Warrant, the kind and amount of shares of stock, other securities, money and
property receivable upon such reorganization, merger or sale by the Holder of
the number of shares of Common Stock then purchasable under this Warrant. Such
new warrant shall provide for adjustments that shall be as nearly equivalent as
may be



                                      -2-
<PAGE>


practicable to the adjustments provided for in this Section 3. The provisions of
this subparagraph (c) shall similarly apply to successive reorganizations,
mergers and sales.

           (d) Certificate of Adjustment. In each case of an adjustment or
readjustment of the Warrant Price, the Company, at its own expense, shall cause
its Chief Financial Officer to compute such adjustment or readjustment in
accordance with the provisions hereof and prepare a certificate showing such
adjustment or readjustment, and shall mail such certificate, by first class
mail, postage prepaid, to the Holder. The certificate shall set forth such
adjustment or readjustment, showing in detail the facts upon which such
adjustment or readjustment is based. No adjustment of the Warrant Price shall be
required to be made unless it would result in an increase or decrease of at
least one cent, but any adjustments not made because of this sentence shall be
carried forward and taken into account in any subsequent adjustment otherwise
required hereunder.

           (e) Adjustment to Number of Shares of Warrant Stock. In the event the
Warrant Price is adjusted under any provision of this Section 3, the number of
shares of Warrant Stock shall be simultaneously adjusted by multiplying the
number of shares of Warrant Stock by a fraction, the numerator of which is the
Warrant Price in effect immediately prior to such adjustment and the denominator
of which is the Warrant Price in effect immediately after such adjustment.

           (f) No Impairment. The Company shall not, by amendment of its Amended
and Restated Certificate of Incorporation or through a reorganization, transfer
of assets, consolidation, merger, dissolution, issue, or sale of securities or
any other voluntary action, avoid or seek to avoid the observance or performance
of any of the terms to be observed or performed under this Warrant by the
Company, but shall at all times in good faith assist in carrying out all of the
provisions of this Section 3 and in taking all such action as may be necessary
or appropriate to protect the Holder's rights under this Section 3 against
impairment. If the Company takes any action affecting the Common Stock other
than as described above that adversely affects the Holder's rights under this
Warrant, the Company shall appropriately adjust the terms of this Warrant,
including, if appropriate, by effecting a downward adjustment of the Warrant
Price, to protect the Holder's rights against such impairment.

           (g) Fractional Shares. No fractional shares shall be issuable upon
exercise or conversion of the Warrant and the number of shares to be issued
shall be rounded down to the nearest whole share. If a fractional share interest
arises upon any exercise or conversion of the Warrant, the Company shall
eliminate such fractional share interest by paying the Holder an amount computed
by multiplying the fractional interest by the fair market value of a full share.

        4. No Shareholder Rights. This Warrant, by itself, as distinguished from
any shares purchased hereunder, shall not entitle the Holder to any of the
rights of a shareholder of the Company.

        5. Reservation of Stock. On and after the date hereof, the Company will
reserve from its authorized and unissued Common Stock a sufficient number of
shares to provide for the issuance of Warrant Stock upon the exercise or
conversion of this Warrant. Issuance of this Warrant shall constitute full
authority to the Company's officers who are charged with the duty



                                      -3-
<PAGE>

of executing stock certificates to execute and issue the necessary certificates
for shares of Warrant Stock issuable upon the exercise or conversion of this
Warrant.

        6. Exercise of Warrant. This Warrant may be exercised as a whole or part
by the Holder, at any time after the date hereof prior to the termination of
this Warrant, by the surrender of this Warrant, together with the Notice of
Exercise and Investment Representation Statement in the forms attached hereto as
Attachments 1 and 2, respectively, duly completed and executed at the principal
office of the Company, specifying the portion of the Warrant to be exercised and
accompanied by payment in full of the Warrant Price in cash or by check with
respect to the shares of Warrant Stock being purchased. This Warrant shall be
deemed to have been exercised immediately prior to the close of business on the
date of its surrender for exercise as provided above, and the person entitled to
receive the shares of Warrant Stock issuable upon such exercise shall be treated
for all purposes as the holder of such shares of record as of the close of
business on such date. As promptly as practicable after such date, the Company
shall issue and deliver to the person or persons entitled to receive the same a
certificate or certificates for the number of full shares of Warrant Stock
issuable upon such exercise. If this Warrant shall be exercised for less than
the total number of shares of Warrant Stock then issuable upon exercise,
promptly after surrender of this Warrant upon such exercise, the Company will
execute and deliver a new warrant, dated the date hereof, evidencing the right
of the Holder to the balance of this Warrant Stock purchasable hereunder upon
the same terms and conditions set forth herein.

        7. Conversion. In lieu of exercising this Warrant or any portion hereof,
at any time the Holder hereof shall have the right to convert this Warrant or
any portion hereof into Warrant Stock by executing and delivering to the Company
at its principal office the written Notice of Conversion and Investment
Representation Statement in the forms attached hereto as Attachments 2 and 3,
specifying the portion of the Warrant to be converted, and accompanied by this
Warrant. The number of shares of Warrant Stock to be issued to the Holder upon
such conversion shall be computed using the following formula:


                                 X=(P)(Y)(A-B)/A

        where X = the number of shares of Common Stock to be issued to the
                  Holder for the portion of the Warrant being converted.

                  P = the portion of the Warrant being converted expressed as a
                      decimal fraction.

                  Y = the total number of shares of Common Stock issuable upon
                      exercise of the Warrant in full.

                  A = the fair market value of one share of Warrant Stock which
                      means (i) the fair market value of the Warrant Stock as of
                      the last Business Day immediately prior to the date the
                      notice of conversion is received by the Company, as
                      reported in the principal market for such securities or,
                      if no such market exists, as determined in good faith by
                      the Company's Board of Directors, or (ii) if this Warrant
                      is being converted in conjunction with a public




                                      -4-
<PAGE>

                      offering of stock the price to the public per share
                      pursuant to the offering.

                  B = the Warrant Price on the date of conversion.

Any portion of this Warrant that is converted shall be immediately canceled.
This Warrant or any portion hereof shall be deemed to have been converted
immediately prior to the close of business on the date of its surrender for
conversion as provided above, and the person entitled to receive the shares of
Warrant Stock issuable upon such conversion shall be treated for all purposes as
the holder of such shares of record as of the close of business on such date. As
promptly as practicable after such date, the Company shall issue and deliver to
the person or persons entitled to receive the same a certificate or certificates
for the number of full shares of Warrant Stock issuable upon such conversion. If
the Warrant shall be converted for less than the total number of shares of
Warrant Stock then issuable upon conversion, promptly after surrender of the
Warrant upon such conversion, the Company will execute and deliver a new
warrant, dated the date hereof, evidencing the right of the Holder to the
balance of the Warrant Stock purchasable hereunder upon the same terms and
conditions set forth herein. If this Warrant is converted, as a whole or in
part, after the occurrence of an event as to which Section 3(c) is applicable,
the Holder shall receive the consideration contemplated by Section 3(c) in lieu
of Common Stock of the Company.

        8. Transfer of Warrant. This Warrant may be transferred or assigned by
the Holder hereof in whole or in part, provided that the transferor provides, at
the Company's request, an opinion of counsel satisfactory to the Company that
such transfer does not require registration under the Securities Act and the
securities law applicable with respect to any other applicable jurisdiction.

        9. Payment of Taxes. The Company will pay all documentary stamp taxes
attributable to the issuance of Warrant Stock upon the exercise of this Warrant;
provided, however, that the Company shall not be required to pay any tax which
may be payable in respect of any transfer involved in the registration of any
certificates for Warrant Stock or Warrants in a name other than that of the
Holder. The Holder shall be responsible for all other tax liability that may
arise as a result of holding or transferring this Warrant or receiving Warrant
Stock upon exercise thereof.

        10. Replacement of Warrant. If this Warrant is mutilated, lost, stolen
or destroyed, the Company shall issue or cause to be issued in exchange and
substitution for and upon cancellation hereof, or in lieu of and substitution
for this Warrant, a new warrant to purchase Common Stock, in substantially the
same form of this Warrant, upon receipt of notice, in writing, of such loss,
theft, destruction and indemnity.

        11. Termination. This Warrant shall terminate on 5:00 p.m. Pacific time
on the Termination Date.

        12. Miscellaneous. This Warrant shall be governed by the laws of the
Commonwealth of Virginia, as such laws are applied to contracts to be entered
into and performed entirely in Virginia by Virginia residents. In the event of
any dispute among the Holder and the Company




                                      -5-
<PAGE>

arising out of the terms of this Warrant, the parties hereby consent to the
exclusive jurisdiction of the federal and state courts located in the
Commonwealth of Virginia for resolution of such dispute, and agree not to
contest such exclusive jurisdiction or seek to transfer any action relating to
such dispute to any other jurisdiction. The headings in this Warrant are for
purposes of convenience and reference only, and shall not be deemed to
constitute a part hereof. Neither this Warrant nor any term hereof may be
changed or waived orally, but only by an instrument in writing signed by the
Company and the Holder of this Warrant. All notices and other communications
from the Company to the Holder of this Warrant shall be delivered personally or
by facsimile transmission or mailed by first class mail, postage prepaid, to the
address or facsimile number furnished to the Company in writing by the last
Holder of this Warrant who shall have furnished an address or facsimile number
to the Company in writing, and if mailed shall be deemed given three days after
deposit in the United States mail.

        Dated as of: April __, 2002




                                        LYNX THERAPEUTICS, INC.

                                        By:
                                           ----------------------------------

                                        Name:
                                             --------------------------------

                                        Title:
                                              -------------------------------




                                      -6-
<PAGE>

                                  Attachment 1



NOTICE OF EXERCISE

TO: ___________________

        1. The undersigned hereby elects to purchase _______________ shares of
the Warrant Stock of Lynx Therapeutics, Inc. pursuant to the terms of the
attached Warrant, and tenders herewith payment of the purchase price in full,
together with all applicable transfer taxes, if any.

        2. Please issue a certificate or certificates representing said shares
of Warrant Stock in the name of the undersigned or in such other name as is
specified below:



                         ------------------------------
                                     (Name)

                         ------------------------------
                                    (Address)

------------------------------                  -------------------------------
(Date)                                          (Name of Warrant Holder)


                                                By:
                                                   ----------------------------

                                                Title:
                                                      -------------------------



<PAGE>

                                  Attachment 2

                       INVESTMENT REPRESENTATION STATEMENT

                           Shares of the Common Stock
                     (as defined in the attached Warrant) of
                             Lynx Therapeutics, Inc.


        In connection with the purchase of the above-listed securities, the
undersigned hereby represents to Lynx Therapeutics, Inc. (the "Company") as
follows:

        (a) The securities to be received upon the exercise of the Warrant (the
"Securities") will be acquired for investment for its own account, not as a
nominee or agent, and not with a view to the sale or distribution of any part
thereof, and the undersigned has no present intention of selling, granting
participation in or otherwise distributing the same, but subject, nevertheless,
to any requirement of law that the disposition of its property shall at all
times be within its control. By executing this statement, the undersigned
further represents that it does not have any contract, undertaking, agreement or
arrangement with any person to sell, transfer, or grant participations to such
person or to any third person, with respect to any Securities issuable upon
exercise of the Warrant.

        (b) The undersigned understands that the Securities issuable upon
exercise of the Warrant at the time of issuance may not be registered under the
Securities Act of 1933, as amended (the "Securities Act"), and applicable state
securities laws, on the ground that the issuance of such securities is exempt
pursuant to Section 4(2) of the Securities Act and state law exemptions relating
to offers and sales not by means of a public offering, and that the Company's
reliance on such exemptions is predicated on the undersigned's representations
set forth herein.

        (c) The undersigned agrees that in no event will it make a disposition
of any Securities acquired upon the exercise of the Warrant unless and until (i)
it shall have notified the Company of the proposed disposition and shall have
furnished the Company with a statement of the circumstances surrounding the
proposed disposition, and (ii) it shall have furnished the Company with an
opinion of counsel satisfactory to the Company and Company's counsel to the
effect that (A) appropriate action necessary for compliance with the Securities
Act and any applicable state securities laws has been taken or an exemption from
the registration requirements of the Securities Act and such laws is available,
and (B) the proposed transfer will not violate any of said laws.

        (d) The undersigned acknowledges that an investment in the Company is
highly speculative and represents that it is able to fend for itself in the
transactions contemplated by this statement, has such knowledge and experience
in financial and business matters as to be capable of evaluating the merits and
risks of its investments, and has the ability to bear the economic risks
(including the risk of a total loss) of its investment. The undersigned
represents that it has had the opportunity to ask questions of the Company
concerning the Company's business and assets and to obtain any additional
information which it considered necessary to verify the



<PAGE>

accuracy of or to amplify the Company's disclosures, and has had all questions
which have been asked by it satisfactorily answered by the Company

        (e) The undersigned acknowledges that the Securities issuable upon
exercise or conversion of the Warrant must be held indefinitely unless
subsequently registered under the Securities Act or an exemption from such
registration is available. The undersigned is aware of the provisions of Rule
144 promulgated under the Securities Act which permit limited resale of shares
purchased in a private placement subject to the satisfaction of certain
conditions, including, among other things, the existence of a public market for
the shares, the availability of certain current public information about the
Company, the resale occurring not less than one year after a party has purchased
and paid for the security to be sold from the Company or any affiliate of the
Company, the sale being through a "broker's transaction" or in transactions
directly with a "market maker" (as provided by Rule 144(f)) and the number of
shares being sold during any three month period not exceeding specified
limitations.


        Dated:
              -----------------------



                                           ------------------------------------
                                           (Typed or Printed Name)

                                           By:
                                              ---------------------------------
                                              (Signature)

                                           ------------------------------------
                                           (Title)



                                       -2-
<PAGE>

                                  Attachment 3


NOTICE OF CONVERSION

TO:
    ---------------------


        1. The undersigned hereby elects to acquire _______________ shares of
the Warrant Stock of Lynx Therapeutics, Inc. pursuant to the terms of the
attached Warrant, by conversion of _________ percent (_____%) of the Warrant.

        2. Please issue a certificate or certificates representing said shares
of Warrant Stock in the name of the undersigned or in such other name as is
specified below:


                         ------------------------------
                                     (Name)

                         ------------------------------
                                    (Address)


----------------------------         ------------------------------------------
(Date)                               (Name of Warrant Holder)

                                     By:
                                        ---------------------------------------

                                     Title:
                                           ------------------------------------
                                     (Title and signature of authorized person)