<PAGE>

                                                                    EXHIBIT 4.2


NEITHER THIS WARRANT NOR THE SECURITIES ISSUABLE UPON EXERCISE HEREOF HAVE BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"),
OR THE SECURITIES LAWS OF ANY STATE.  SUCH SECURITIES MAY NOT BE SOLD OR
OTHERWISE DISPOSED OF UNLESS PURSUANT TO A REGISTERED OFFERING OR BY TRANSFER
EXEMPT FROM REGISTRATION UNDER THE SECURITIES ACT AND APPLICABLE STATE
SECURITIES LAWS.

                         ENCHIRA BIOTECHNOLOGY CORPORATION

                            COMMON STOCK PURCHASE WARRANT

No. W-00-______                                                     ______Shares

     This certifies that, for value received, _________________________ or
registered assigns (the "holder"), upon due exercise of this Warrant, is
entitled to purchase from Enchira Biotechnology Corporation, a Delaware
corporation (the "Company"), at any time on or after September 8, 2000 (the
"Initial Exercise Date"), and before the close of business on September 8, 2002,
or if not a trading date on the New York Stock Exchange, the next following
trading date (the "Expiration Date"), all or any part of _______ fully paid and
nonassessable Shares (the "Warrant Shares") of the Common Stock, par value $0.01
per share, of the Company ("Common Stock"), at a purchase price of $7.44 per
share (the "Purchase Price"), both the Purchase Price and the number of Warrant
Shares issuable upon exercise of this Warrant being subject to possible
adjustment as provided below.

     This Warrant is hereinafter called the "Warrant." The holder hereof and
all subsequent holders of this Warrant shall be entitled to all rights and
benefits provided to the holder or holders hereof pursuant to the terms of this
Warrant.

     SECTION 1.  EXERCISE OF WARRANT.  (a) The holder of this Warrant may, at
any time on or after the Initial Exercise Date and on or before the Expiration
Date, exercise this Warrant in whole at any time or in part (but not less than
1,000 Warrant Shares) from time to time for the purchase of the Warrant Shares
or other securities which such holder is then entitled to purchase hereunder
("Warrant Securities") at the Purchase Price (as hereinafter defined).  In order
to exercise this Warrant in whole or in part, the holder hereof shall deliver to
the Company (i) a written notice of such holder's election to exercise this
Warrant, which notice shall specify the number of Warrant Shares to be
purchased, (ii) payment of the aggregate purchase price of the Warrant Shares
being purchased by certified or bank cashier's check, unless pursuant to a
Cashless Exercise as described in subsection (b) below, and (iii) this Warrant,
provided that, if such Warrant Shares or other Warrant Securities have not then
been registered under the Securities Act or applicable state securities laws,
the Company may require that such holder furnish to the Company a written
statement that such holder is purchasing such Warrant Shares or other Warrant
Securities for such holder's own account for investment and not with a view to
the distribution thereof, that none of such shares will be offered or sold in
violation of the provisions of the Securities Act and applicable state
securities laws and as to such other matters relating to the holder as the
Company may reasonably request to permit the issuance of such Warrant Shares or
other Warrant Securities without registration under the Securities Act and
applicable state securities laws.  Upon receipt thereof, the Company shall, as
promptly as practicable, execute or cause to be executed and deliver to such
holder a certificate or certificates representing the aggregate number of
Warrant Shares (or if applicable, other Warrant Securities) specified in said
notice.  The stock certificate or certificates so delivered shall be in the
denomination of 100 shares each or such other denominations

<PAGE>


as may be specified in said notice and shall be registered in the name of
such holder or such other name as shall be designated in said notice.

     No fractional Warrant Shares are to be issued upon the exercise of this
Warrant, but the Company shall pay a cash adjustment in respect of any fraction
of a share which would otherwise be issuable in an amount equal to the same
fraction of the fair market value per share of the Warrant Shares on the day of
exercise, as reasonably determined by the Company consistent with the
determination of "Current Price" below.  If this Warrant shall have been
exercised only in part, the Company shall, at the time of delivery of said
certificate or certificates, deliver to such holder a new Warrant evidencing the
rights of such holder to purchase the remaining Warrant Shares called for by
this Warrant, which new Warrant shall in all other respects be identical with
this Warrant, or, at the request of such holder, appropriate notation may be
made on this Warrant and same returned to such holder.  The Company shall pay
all expenses, taxes and other charges payable in connection with the
preparation, execution and delivery of share certificates under this Section,
except that, if such share certificates are requested to be registered in a name
or names other than the name of the holder of this Warrant, funds sufficient to
pay all stock transfer taxes which shall be payable upon the execution and
delivery of such share certificates shall be paid by the holder hereof at the
time of delivering the notice of exercise mentioned above.

     The Company represents, warrants and agrees that it shall at all times
prior to the exercise of this Warrant reserve sufficient shares of Common Stock
for issuance upon the exercise hereof, and all Warrant Shares issuable upon any
exercise of this Warrant in accordance herewith shall be validly authorized and
issued, fully paid and nonassessable.

     This Warrant shall not entitle the holder hereof to any of the rights of
a stockholder of the Company prior to exercise in the manner herein provided.

     (b)  Notwithstanding anything in subsection (a) to the contrary, the
holder of this Warrant may elect to exercise this Warrant in part (but not for
less than 1,000 Warrant Shares) or in whole, at any time on or after the Initial
Exercise Date and on or before the Expiration Date, by the surrender of this
Warrant (with the cashless exercise form at the end hereof duly executed) (a
"Cashless Exercise") at the address set forth in subsection 6(i) hereof.  Such
presentation and surrender shall be deemed a waiver of the holder's obligation
to pay the Purchase Price, or the proportionate part thereof if this Warrant is
exercised in part.  In the event of a Cashless Exercise, the Holder shall
exchange its Warrant for that number of Warrant Shares or Warrant Securities, as
the case may be, subject to such Cashless Exercise multiplied by a fraction, the
numerator of which shall be the difference between the then Current Market Price
Per Share of the Common Stock and the Per Share Purchase Price, and the
denominator of which shall be the then Current Market Price Per Share of the
Common Stock.  For purposes of any computation under this subsection, the then
Current Market Price shall be based on the trading day prior to the Cashless
Exercise.  "Current Market Price" shall be deemed to be the last sale price of
the Common Stock on the trading day prior to such date or, in case no such
reported sales take place on such day, the average of the last reported bid and
asked prices of the Common Stock on such day, in either case on the principal
national securities exchange on which the Common Stock is admitted to trading or
listed, or if not listed or admitted to trading on any such exchange, the
representative closing bid price of the Common Stock as reported by the National
Association of Securities Dealers, Inc. Automated Quotations System ("NASDAQ"),
or other similar organization if NASDAQ is no longer reporting such information,
or, if the Common Stock is not reported on NASDAQ, the high per share bid price
for the Common Stock in the over-the-counter market as reported by the National
Quotation Bureau or similar organization, or if not so available, the fair


                                     -2-
<PAGE>


market value of the Common Stock as determined in good faith by the Board of
Directors, subject to the right of the holder to contest and appoint an
appraiser to make such determination.

     SECTION 2.  TRANSFER, DIVISION AND COMBINATION.  The Company shall keep
at its principal executive office a register for the registration of, and
registration of transfers of, the Warrants.  The name and address of each holder
of one or more Warrants, each transfer thereof and the name and address of each
transferee of one or more Warrants shall be registered in such register.  Prior
to due presentment for registration of transfer, the person in whose name any
Warrants shall be registered shall be deemed and treated as the owner and holder
thereof for all purposes hereof, and the Company shall not be affected by any
notice or knowledge to the contrary.  The Company shall give to any holder of a
Warrant promptly upon request therefor, a complete and correct copy of the names
and addresses of all registered holders of Warrants.

     Subject to the provisions of Section 3, upon surrender of any Warrant at
the principal executive office of the Company for registration of transfer or
exchange (and in the case of a surrender for registration of transfer, duly
endorsed or accompanied by a written instrument of transfer duly executed by the
registered holder of such Warrant or his attorney duly authorized in writing and
accompanied by the address for notices of each transferee of such Warrant or
part thereof), the Company shall execute and deliver, at the Company's expense,
one or more new Warrants (as requested by the holder thereof) in exchange
therefor, exercisable for an aggregate number of Warrant Shares equal to the
number of shares for which the surrendered Warrant is exercisable and issued to
such person or persons as such holder may request, which Warrant or Warrants
shall in all other respects be identical with this Warrant.

     Upon receipt by the Company of evidence reasonably satisfactory to it of
the ownership of and the loss, theft, destruction or mutilation of any Warrant,
and (a) in the case of loss, theft or destruction, of indemnity reasonably
satisfactory to it (provided that if the holder of such Warrant is, or is a
nominee for, an original holder, such person's own unsecured agreement of
indemnity shall be deemed to be satisfactory), or (b) in the case of mutilation,
upon surrender and cancellation thereof, the Company at its own expense shall
execute and deliver, in lieu thereof, a new Warrant identical in all respects to
such lost, stolen, destroyed or mutilated Warrant.

     SECTION 3.  COMPLIANCE WITH SECURITIES ACT; RESTRICTIONS ON TRANSFER AND
SALE.  (a)   Each certificate for Warrant Shares (or other Warrant Securities)
initially issued upon the exercise of this Warrant and each certificate for
Warrant Shares (or other Warrant Securities) issued to subsequent transferees of
any such certificate shall (unless otherwise permitted by this Section 3) be
stamped or otherwise imprinted with legend in substantially the following form:

"THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT") OR
THE SECURITIES LAWS OF ANY STATE.  SUCH SECURITIES MAY NOT BE SOLD OR
OTHERWISE DISPOSED OF UNLESS PURSUANT TO A REGISTERED OFFERING OR BY
TRANSFER EXEMPT FROM REGISTRATION UNDER THE SECURITIES ACT AND APPLICABLE
STATE SECURITIES LAWS."

     (b)  The holder understands that Warrant Shares (or other Warrant
Securities) which may be acquired by it upon exercise of this Warrant shall be
entitled to certain registration rights provided for in the


                                     -3-
<PAGE>


Subscription Agreement relating to the purchase and issuance of the Shares
and this Warrant between the Company and the holder.

     SECTION 4. ADJUSTMENT OF PURCHASE PRICE.

     (a)  The Purchase Price and the number of Warrant Shares and the number
or amount of any other securities and property as hereinafter provided for which
this Warrant may be exercisable shall be subject to adjustment from time to time
effective upon each occurrence of any of the following events.

               (i)    If the Company shall declare or pay any dividend
          with respect to its Common Stock payable in shares of Common
          Stock, subdivide the outstanding Common Stock into a greater
          number of shares of Common Stock, or reduce the number of shares
          of Common Stock outstanding (by stock split, reverse stock split,
          reclassification or otherwise than by repurchase of its Common
          Stock) (any of such events being hereinafter called a "Stock
          Split"), the Purchase Price and number of Warrant Shares issuable
          upon exercise of this Warrant shall be appropriately adjusted so
          as to entitle the holder hereof to receive upon exercise of this
          Warrant, for the same aggregate consideration provided herein, the
          same number of shares of Common Stock (plus cash in lieu of
          fractional shares) as the holder would have received as a result
          of such Stock Split had such holder exercised this Warrant in full
          immediately prior to such Stock Split.

               (ii)   If the Company shall merge or consolidate with or
          into one or more corporations or partnerships and the Company is
          the sole surviving corporation, or the Company shall adopt a plan
          of recapitalization or reorganization in which the Common Stock is
          exchanged for or changed into another class of stock or other
          security or property of the Company, the holder of this Warrant
          shall, for the same aggregate consideration provided herein, be
          entitled upon exercise of this Warrant to receive in lieu of the
          number of shares of Common Stock as to which this Warrant would
          otherwise be exercisable, the number of shares of Common Stock or
          other securities (plus cash in lieu of fractional shares) or
          property to which such holder would have been entitled pursuant to
          the terms of the agreement or plan of merger, consolidation,
          recapitalization or reorganization had such holder exercised this
          Warrant in full immediately prior to such merger, consolidation,
          recapitalization or reorganization.

               (iii)  If the Company is merged or consolidated with or
          into one or more corporations or partnerships under circumstances
          in which the Company is not the sole surviving corporation, or if
          the Company sells or otherwise disposes of substantially all its
          assets, and in connection with any such merger, consolidation or
          sale the holders of Common Stock receive stock or other securities
          convertible into equity of the surviving or acquiring corporations
          or entities, or other securities or property after the effective
          date of such merger, consolidation or sale, as the case may be,
          the holder of this Warrant shall, for the same aggregate
          consideration provided herein, be entitled upon exercise of this
          Warrant to receive, in lieu of the shares of Common Stock as to
          which this Warrant would otherwise be exercisable, shares of such
          stock or other securities (plus cash in lieu of fractional shares)
          or property as the holder of this Warrant would have received
          pursuant to the terms of the merger, consolidation or sale had
          such holder exercised this Warrant in full immediately prior to
          such merger, consolidation or sale.  In the event of any
          consolidation, merger or sale as described in this Section 4(d),
          provision shall be made in connection therewith for the surviving
          or acquiring corporations or partnerships to assume all
          obligations and duties of the


                                     -4-
<PAGE>


          Company hereunder or to issue substitute warrants in lieu of this
          Warrant with all such changes and adjustments in the number or kind
          of shares of stock or securities or property thereafter subject to
          this Warrant or in the Purchase Price as shall be required in
          connection with this Section 4(d).

               (iv)   If the Company (other than in connection with a sale
          described in Section 4(d)) proposes to liquidate and dissolve, the
          Company shall give notice thereof as provided in Section 5(b)
          hereof and shall permit the holder of this Warrant to exercise any
          unexercised portion hereof at any time within the 10 day period
          following delivery of such notice, if such holder should elect to
          do so, and participate as a stockholder of the Company in
          connection with such dissolution.

     (b)  Whenever any adjustment is made as provided in any provision of
this Section 4:

               (i)    the Company shall compute the adjustments in
          accordance with this Section 4 and shall prepare a certificate
          signed by an officer of the Company setting forth the adjusted
          number of shares or other securities or property and Purchase
          Price, as applicable, and showing in reasonable detail the facts
          upon which such adjustment is based, and such certificate shall
          forthwith be filed with the Company or its designee; and

               (ii)   a notice setting forth the adjusted number of shares
          or other securities or property and the Purchase Price, as
          applicable, shall forthwith be required, and as soon as
          practicable after it is prepared, such notice shall be delivered
          by the Company to the holder of record of each Warrant.

     (c)  If at any time, as a result of any adjustment made pursuant to
this Section 4, the holder of this Warrant shall become entitled, upon exercise
hereof, to receive any shares other than shares of Common Stock or to receive
any other securities, the number of such other shares or securities 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 contained in this Section 4 with respect to the Common Stock.

     SECTION 5.  SPECIAL AGREEMENTS OF THE COMPANY.

     (a)  The Company covenants and agrees that it will reserve and set
apart and have at all times a number of shares of authorized but unissued Common
Stock (and, if applicable, other Warrant Securities) then deliverable upon the
exercise of the Warrants or any other rights or privileges provided for therein
sufficient to enable it at any time to fulfill all its obligations thereunder;
and if at any time the number of authorized but unissued shares of Common Stock
shall not be sufficient to effect the exercise of this Warrant at the Purchase
Price then in effect, the Company will take such corporate action as may, in the
reasonable opinion of its counsel, be necessary to increase its authorized
shares but unissued shares of Common Stock (and, if applicable, other Warrant
Securities) to such number of shares as shall be sufficient for such purposes.

     (b)  In case the Company proposes

               (i)    to pay any dividend upon the Common Stock or make
          any distribution or offer any subscription or other rights to the
          holders of Common Stock, or

               (ii)   to effect any capital reorganization or reclassification
          of capital stock of the Company, or


                                     -5-
<PAGE>


               (iii)  to effect the consolidation, merger, sale of all or
          substantially all of the assets, liquidation, dissolution or winding
          up of the Company,

then the Company shall cause notice of any such intended action to be given to
each holder of the Warrants not less than 15 nor more than 60 days prior to the
date on which the transfer books of the Company shall close or a record be taken
for such dividend or distribution, or the date when such capital reorganization,
reclassification, consolidation, merger, sale, liquidation, dissolution or
winding up shall be effected, or the date of such other event, as the case may
be.

     SECTION 6.  NOTICES.  Any notice or other document required or permitted
to be given or delivered to holders of Warrants and holders of Common Stock (or
other Warrant Securities) shall be in writing and sent (a) by telecopy if the
sender on the same day sends a confirming copy of such notice by a recognized
overnight delivery service (charges prepaid), or (b) by registered or certified
mail with return receipt requested (postage prepaid) or (c) by a recognized
overnight delivery service (with charges prepaid).


          (i)  if to the Company, at Enchira Biotechnology Corporation,
     4200 Research Forest Drive, The Woodlands, Texas 77381, Telecopy No.:
     (281) 364-6112, or such other address as it shall have specified to the
     holders of Warrants in writing; or

          (ii) if to a holder, at its address set forth below, or such
     other address as it shall have specified to the Company in writing.

Notices given under this Section 6 shall be deemed given only when actually
received.

     SECTION 7.   AMENDMENT.  This Warrant may not be amended, modified or
otherwise altered in any respect except by the written consent of the registered
holder of this Warrant and the Company.

     SECTION 8.   SUCCESSORS AND ASSIGNS.  This Warrant shall be binding upon
and inure to the benefit of the Company and the holder of this Warrant and their
respective successors and permitted assigns.

     SECTION 9.   GOVERNING LAW.  This Warrant shall be governed by and
construed in accordance with the laws of the State of Delaware, without
reference to the conflicts of law principles thereof.

          IN WITNESS WHEREOF, the Company has caused this Warrant to be
signed in its name by its duly authorized officers and accepted by the holder of
this Warrant this 8th day of September, 2000.


ATTEST:                              ENCHIRA BIOTECHNOLOGY CORPORATION

By:                                  By:
   --------------------------           --------------------------------------
Name:   Paul G. Brown, III               Peter P. Policastro
Title:  Secretary                        President and Chief Executive Officer


Address for Notices:

     -----------------------------

     -----------------------------



                                     -6-
<PAGE>


                                    SUBSCRIPTION

          The undersigned, ___________________, pursuant to the provisions
of the foregoing Warrant, hereby agrees to subscribe for and purchase
____________________ shares of the Common Stock, par value $.01 per share, of
Enchira Biotechnology Corporation covered by said Warrant, and makes payment
therefor in full at the price per share provided by said Warrant.

Dated:                                 Signature:
      ------------------                         ----------------------

Signature Guarantee:                   Address:
                                               ------------------------

                                       Social Security No.
------------------------                                  -------------





                                 CASHLESS EXERCISE

          The undersigned ___________________, pursuant to the provisions of
the foregoing Warrant, hereby elects to exchange its Warrant for
___________________ shares of Common Stock, par value $.01 per share, of Enchira
Biotechnology Corporation pursuant to the Cashless Exercise provisions of the
Warrant.

Dated:                                 Signature:
      ------------------                         ----------------------

Signature Guarantee:                   Address:
                                               ------------------------

                                       Social Security No.
------------------------                                  -------------



                                     ASSIGNMENT

          FOR VALUE RECEIVED _______________ hereby sells, assigns and
transfers unto ____________________ (SS#_________________) the foregoing Warrant
and all rights evidenced thereby, and does irrevocably constitute and appoint
_____________________, attorney, to transfer said Warrant on the books of
Enchira Biotechnology Corporation.


Dated:                                 Signature:
      ------------------                         ----------------------

Signature Guarantee:                   Address:
                                               ------------------------


------------------------


                                     -7-
<PAGE>


                                 PARTIAL ASSIGNMENT


FOR VALUE RECEIVED _______________ hereby assigns and transfers unto
____________________ (SS#________________) the right to purchase _______ shares
of the Common Stock, par value $.01 per share, of Enchira Biotechnology
Corporation covered by the foregoing Warrant, and a proportionate part of said
Warrant and the rights evidenced thereby, and does irrevocably constitute and
appoint ____________________, attorney, to transfer that part of said Warrant on
the books of Enchira Biotechnology Corporation.


Dated:                                 Signature:
      ------------------                         ----------------------

Signature Guarantee:                   Address:
                                               ------------------------


------------------------






                                     -8-