<PAGE>

                                                                 Exhibit 4.2
                                                                 -----------


         This security and the securities issuable upon exercise hereof have
not been registered under the Securities Act of 1933, as amended, or any
state securities law and may not be transferred, sold or offered for sale
unless registered pursuant to such Act and any applicable state securities
law or unless an exemption from such registration is available.


                                                         Dated: May 11, 2001


                                   WARRANT
                  TO PURCHASE 12,500 SHARES OF COMMON STOCK
                                     OF
                           ZOLTEK COMPANIES, INC.


EXPIRING MAY 11, 2006


         THIS IS TO CERTIFY THAT, for value received, SOUTHWEST BANK OF ST.
LOUIS, or its registered successors or assigns (the "Holder"), is entitled
to purchase from Zoltek Companies, Inc., a Missouri corporation (the
"Company"), at any time or from time to time after 9:00 a.m., St. Louis,
Missouri time, on the date hereof and prior to 5:00 p.m., St. Louis,
Missouri time, on May 11, 2006 (the "Expiration Date"), at the principal
executive offices of the Company, at the Exercise Price, Twelve Thousand
Five Hundred (12,500) shares of Common Stock, par value $0.01 per share (the
"Common Stock"), of the Company, all subject to adjustment and upon the
terms and conditions as hereinafter provided, and is entitled also to
exercise the other appurtenant rights, powers and privileges hereinafter
described.

         Certain terms used in this Warrant are defined in Article V hereof.
                                                           ---------

                                  ARTICLE I
                             EXERCISE OF WARRANT

         1.1      Method of Exercise. To exercise this Warrant in whole or
                  ------------------
in part, the Holder shall deliver to the Company (a) this Warrant, (b) a
written notice, in substantially the form of the Subscription Notice
attached hereto (the "Exercise Notice"), of such Holder's election to
exercise this Warrant, which notice shall specify the number of shares of
Common Stock to be purchased, the denominations of the share certificate or
certificates desired and the name or names in which such certificates are to
be registered, and (c) payment of the Exercise Price with respect to such
shares. Such payment may be made, at the option of the Holder, by cash,
certified or bank cashier's check or wire transfer.

         The Company shall, as promptly as practicable and in any event
within ten (10) days after receipt of the Exercise Notice, execute and
deliver or cause to be executed and delivered, in accordance with the
Exercise Notice, a certificate or certificates representing the aggregate
number of shares of Common Stock specified in said notice. The share
certificate or certificates so delivered shall be in such denominations as
may be specified in such notice or, if such notice shall not specify
denominations, in


                                     1

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denominations of 1,000 shares each, and shall be issued in the name of the
Holder or such other name or names as shall be designated in such notice.
Such certificate or certificates shall be deemed to have been issued, and
such Holder or any other Person so designated to be named therein, shall be
deemed for all purposes to have become a holder of record of such shares as
of the date the aforementioned notice is received by the Company. If this
Warrant shall have been exercised only in part, the Company shall, at the
time of delivery of the certificate or certificates, deliver to the Holder a
new warrant evidencing the rights to purchase the remaining shares of Common
Stock called for by this Warrant, which new warrant shall in all other
respects be identical with this Warrant, as it may from time to time be
amended.

         1.2      Shares To Be Fully Paid, Nonassessable and Registered. All
                  -----------------------------------------------------
shares of Common Stock issued upon the exercise of this Warrant shall be
validly issued, fully paid and nonassessable and, if the Common Stock of any
class is then listed on any national securities exchange (as such term is
used in the Exchange Act) or quoted on The Nasdaq Stock Market, upon
issuance all such shares of Common Stock shall be duly listed or quoted
thereon, as the case may be.

         1.3      No Fractional Shares To Be Issued. The Company shall not
                  ---------------------------------
be required to issue fractions of shares of Common Stock upon exercise of
this Warrant. If any fraction of a share would, but for this Section 1.3, be
                                                             -----------
issuable upon any exercise of this Warrant, in lieu of such fractional share
the Company shall pay to the Holder, in cash, an amount equal to the same
fraction of the fair market value per share of outstanding Common Stock on
the Business Day immediately prior to the date of such exercise, as
determined in good faith by the Company's Board of Directors.


                                 ARTICLE II
                TRANSFER, EXCHANGE AND REPLACEMENT OF WARRANT

         2.1      Ownership of Warrant. The Company may deem and treat the
                  --------------------
Person in whose name this Warrant is registered as the holder and owner
hereof (notwithstanding any notations of ownership or writing hereon made by
any Person other than the Company) for all purposes and shall not be
affected by any notice to the contrary, until presentation of this Warrant
for registration of transfer as provided in this Article II.
                                                 ----------

         2.2      Transfer of Warrant. The Company agrees to maintain books
                  -------------------
for the registration of transfers of this Warrant, and transfer of this
Warrant and all rights hereunder shall be registered, in whole or in part,
on such books, upon surrender of this Warrant at the principal executive
offices of the Company, together with a written assignment of this Warrant
duly executed by the Holder or its duly authorized agent or attorney, and
funds sufficient to pay any transfer taxes payable upon such transfer. Upon
surrender, the Company (subject to being satisfied that such transfer is
exempt from registration under the Securities Act) shall execute and deliver
a new warrant or warrants in the name of the assignee or assignees and in
the denominations specified in the instrument of assignment, and this
Warrant shall promptly be cancelled. Notwithstanding the foregoing, this
Warrant may be exercised by a transferee without having a new warrant
issued.

        2.3       Division or Combination of Warrants. This Warrant may be
                  -----------------------------------
divided or combined with other warrants upon surrender hereof and of any
warrant or warrants with which this Warrant is to be combined at the
principal executive offices of the Company, together with a written notice
specifying the names and denominations in which the new warrant or warrants
are to be issued, signed by the holders hereof and thereof or their
respective duly authorized agents or attorneys. Subject to compliance with
Section 2.2 as to any transfer which may be involved in the division or
-----------
combination, the Company shall


                                     2

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execute and deliver a new warrant or warrants in exchange for the Warrants
to be divided or combined in accordance with such notice.

         2.4      Loss, Theft, Destruction of Warrant Certificates. Upon
                  ------------------------------------------------
receipt of evidence satisfactory to the Company of the loss, theft,
destruction or mutilation of any Warrant and, in the case of any such loss,
theft or destruction, upon receipt of an affidavit of loss which shall
include an indemnification provision satisfactory to the Company, or, in the
case of any such mutilation, upon surrender and cancellation of such
Warrant, the Company will make and deliver, in lieu of such lost, stolen,
destroyed or mutilated Warrant, a new warrant of like tenor representing the
right to purchase the same aggregate number of shares of Common Stock.

         2.5      Expenses of Delivery of Warrant. The Company shall pay all
                  -------------------------------
expenses, taxes and other charges payable in connection with the
preparation, issuance and delivery of share certificates of Common Stock
issuable upon exercise of this Warrant and new warrants.

                                 ARTICLE III
                               CERTAIN RIGHTS

         3.1      Delivery of Reports and Information.
                  -----------------------------------

                  (a)      The Company covenants and agrees with the Holder
that the Company will:

                           (i)      Do or cause to be done all things
                  necessary to preserve, renew and keep in full force and
                  effect the Company's legal existence; and

                           (ii)     Upon request of the Holder, furnish
                  to the Holder promptly after the same become publicly
                  available, copies of such registration statements, annual,
                  periodic and other reports, and such proxy statements and
                  other information, if any, as shall be filed by the
                  Company with the Securities and Exchange Commission
                  pursuant to the requirements of the Securities Act or the
                  Exchange Act.

                  (b)      In case at any time the Company shall declare any
dividend on its Common Stock, whether payable in cash, stock or other
property, then the Company shall give written notice to the Holder of the
date on which the books of the Company shall close or a record shall be
taken for such dividend. Such notice shall also specify the date as of which
the holders of Common Stock of record shall participate in such dividend.
Such written notice shall be given at least twenty (20) days prior to the
action in question, and not more than ninety (90) days and not less than
twenty (20) days prior to the relevant record date or the date fixed for
determining stockholders entitled to participate therein, as the case may be.

                                 ARTICLE IV
                           ANTIDILUTION PROVISIONS

         4.1      Adjustments Generally. The Exercise Price and the number
                  ---------------------
of shares of Common Stock (or other securities or property) issuable upon
exercise of this Warrant shall be subject to adjustment from time to time
upon the occurrence of certain events, as provided in this Article IV.
                                                           ----------

         4.2      Stock Reorganization and Number of Shares. The Exercise
                  -----------------------------------------
Price and the number and kind of securities purchasable upon exercise of
this Warrant shall be subject to adjustment from time to time upon the
happening of certain events as follows: In case the Company shall hereafter
(i) declare a


                                     3

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dividend or a distribution on its Common Stock payable in shares of its
Common Stock, (ii) subdivide its outstanding shares of Common Stock, (iii)
combine its outstanding Common Stock into a smaller number of shares, or
(iv) issue other securities of the Company by reclassification of its
Common Stock (including any such reclassification in connection with a
consolidation or merger in which the Company is the continuing corporation),
the number of shares of Common Stock for which this Warrant may be exercised
at the time of the record date for such dividend or distribution or the
effective date of such subdivision, combination or reclassification shall be
proportionately adjusted so that the Holder of any Warrant exercised after
such date shall be entitled to receive the aggregate number and kind of
shares of Common Stock which, if such Warrant had been exercised immediately
prior to such time, the Holder would have owned upon such exercise and been
entitled to receive upon such dividend, distribution, subdivision,
combination or reclassification. Such adjustment shall be made successively
whenever any event listed above shall occur.

         4.3      Merger or Consolidation.
                  -----------------------

                  (a)      In case of any consolidation or merger of the
Company with or into another corporation (other than a merger in which the
Company is the continuing corporation and which does not result in any
reclassification or change of outstanding shares of the Company's capital
stock issuable upon exercise of this Warrant), or in case of any sale or
conveyance to another corporation of the property, assets, business and
goodwill of the Company as an entirety or substantially as an entirety,
then, as a condition of such consolidation, merger, sale or conveyance, the
Company shall cause lawful and adequate provision to be made by the Company
or such successor or purchasing corporation, as the case may be (such
successor or purchasing corporation thereafter being deemed to be the
Company for the purpose hereof) whereby the Holder shall have the right
thereafter (in lieu of any other right hereunder) to exercise this Warrant
for the kind and amount of shares of stock and other securities and property
receivable upon such consolidation, merger, sale or conveyance by a holder
of the number of shares of Common Stock into which this Warrant might have
been exercised immediately prior to such consolidation, merger, sale or
conveyance. Such provision shall include provision for adjustments which
shall be as nearly equivalent as may be practicable to the adjustments
provided for herein. If, as a result of an adjustment made pursuant hereto,
the Holder upon exercise shall become entitled to receive shares of two or
more classes of Common Stock or other capital stock of the Company, the
Board of Directors of the Company (whose determination shall be conclusive)
shall determine the allocation of the adjusted Exercise Price between or
among shares of such classes of capital stock.

                  (b)      The Company shall give the Holder written notice
of any proposed merger of the Company or any other corporate reorganization,
or of any sale of all or substantially all of the assets of the Company.
Such notice shall be given not later than twenty (20) days prior to the
stockholders, meeting called to approve such transaction or twenty (20) days
prior to the closing of such transaction, whichever is earlier. Such notice
shall describe the material terms and conditions of the contemplated
transaction and the Company shall thereafter give the Holders prompt notice
of any material changes in such terms and conditions.

                  (c)      If subsequent to the giving of notice pursuant to
Section 4.3(b) above but not later than ten (10) days prior to the closing
--------------
of such transaction, the Holder elects to exercise this Warrant, then the
exercise may, at the option of such Holder, be conditioned upon the closing
of such transaction, in which event the person(s) entitled to receive the
Common Stock issuable upon exercise of this Warrant shall not be deemed to
have exercised this Warrant until immediately prior to the closing of such
transaction.


                                     4

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         4.4      No Impairment. The Company will not, by amendment of its
                  -------------
Amended and Restated Articles of Incorporation or through any
reorganization, transfer of assets, consolidation, merger, dissolution,
issuance 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 hereunder by the Company but will at all times in good faith
assist in the carrying out of all the provisions of this Article IV and in
                                                         ----------
the taking of all actions that may be necessary or appropriate to protect
the exercise rights of the Holder.

         4.5      Certificate as to Adjustments. Upon the occurrence of each
                  -----------------------------
adjustment or readjustment of the Exercise Price or the number of shares of
Common Stock issuable upon exercise of this Warrant pursuant to this Article
                                                                     -------
IV, the Company at its expense shall promptly compute such adjustment or
--
readjustment in accordance with the terms of this Article IV and furnish to
                                                  ----------
the Holder a certificate setting forth such adjustment or readjustment and
showing in detail the facts upon which such adjustment or readjustment is
based. The Company shall, upon the written request at any time of the
Holder, furnish or cause to be furnished to the Holder a like certificate
setting forth (i) such adjustments and readjustments, (ii) the Exercise
Price in effect at the time, and (iii) the number of shares of Common Stock
and the amount, if any, of other property that would be received at that
time upon exercise of this Warrant.

         4.6      Proceeding Prior to Any Action Requiring Adjustment. As
                  ---------------------------------------------------
a condition precedent to the taking of any action which would require an
adjustment pursuant to this Article IV, the Company shall take any action
                            ----------
which may be necessary, including obtaining regulatory approvals or
exemptions, in order that the Company may thereafter validly and legally
issue as fully paid and nonassessable all shares of Common Stock which the
holders of Warrants are entitled to receive upon exercise thereof.

                                  ARTICLE V
                                 DEFINITIONS

         The following terms, as used in this Warrant, have the following
respective meanings:

         "Business Days" means a day on which the principal national
securities exchange on which the Common Stock of the Company is listed or
admitted to trading is open for business.

         "Exchange Act" means the Securities Exchange Act of 1934, as
amended, and any successor Federal statute, and the rules and regulations of
the Securities and Exchange Commission (or its successor) thereunder, all as
the same shall be in effect at the time.

         "Exercise Price" means Five Dollars ($5.00) per share of Common
Stock, subject to adjustment pursuant to Article IV.
                                         ----------

         "Person" means an individual partnership, corporation, association,
trust, joint venture, unincorporated organization and any government,
governmental department or agency or political subdivision thereof.

         "Securities Act" means the Securities Act of 1933, as amended,
and any successor Federal statute, and the rules and regulations of the
Securities and Exchange Commission (or its successor) thereunder, all as
the same shall be in effect from time to time.


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                                 ARTICLE VI
                                MISCELLANEOUS

         6.1      Notices. Any notice or other communication required by or
                  -------
given in connection with this Warrant shall be deemed to be delivered if in
writing (or in the form of a telecopy) addressed as provided below (or to
such other address or telecopy number as may be provided in writing by the
recipient to the other party hereto) and if (a) actually delivered to said
address (b) telecopied to the number of the recipient set forth below before
5:00 p.m. (local time at the recipient's office) on a Business Day, or (c)
in the case of a letter, three Business Days shall have elapsed after the
same shall have been deposited in the United States mails, postage prepaid
and registered or certified:

         If to Company, at:

                  Zoltek Companies, Inc.
                  3101 McKelvey Road
                  St. Louis, Missouri 63044
                  Telecopy:  (314) 291-9082
                  Attention: Zsolt Rumy

         If to Holder at:

                  Southwest Bank of St. Louis
                  13205 Manchester Road
                  St. Louis, Missouri 63131
                  Telecopy:  (314) 543-3377
                  Attention: Robert J. Witterschein

         6.2      Waivers; Amendments. No failure or delay of the Holder in
                  -------------------
exercising any power or right hereunder shall operate as a waiver thereof,
nor shall any single or partial exercise of such right or power, or any
abandonment or discontinuance of steps to enforce such a right or power,
preclude any other or further exercise thereof or the exercise of any other
right or power. The rights and remedies of the Holder are cumulative and not
exclusive of any rights or remedies which it would otherwise have. The
provisions of this Warrant may be amended, modified or waived with (and only
with) the written consent of the Company and Holder. No notice or demand on
the Company in any case shall entitle the Company to any other or further
notice or demand in similar or other circumstances.

         6.3      Governing Law. This Warrant shall be construed in
                  -------------
accordance with and governed by the internal laws of the State of Missouri.

         6.4      Survival of Agreements; Representations and Warranties,
                  -------------------------------------------------------
etc.. All warranties, representations and covenants made by the Company
----
herein or in any certificate or other instrument delivered by or on behalf
of it in connection with this Warrant shall be considered to have been
relied upon by the Holder and shall survive the issuance and delivery of
this Warrant, regardless of any investigation made by the Holder, and shall
continue in full force and effect so long as this Warrant is outstanding.
All statements in any such certificate or other instrument shall constitute
representations and warranties hereunder.

         6.5      Covenants To Bind Successor and Assigns. All covenants,
                  ---------------------------------------
stipulations, promises and agreements in this Warrant contained by or on
behalf of the Company shall bind its successors and assigns.


                                     6

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         6.6     Severability. In case any one or more of the provisions
                 ------------
contained in this Warrant shall be invalid, illegal or unenforceable in any
respect, the validity, legality and enforceability of the remaining
provisions contained herein shall not in any way be affected or impaired
thereby.

         6.7      Section Headings. The section headings used herein are for
                  ----------------
convenience of reference only, are not part of this Warrant and are not to
affect the construction of or be taken into consideration in interpreting
this Warrant.

         6.8      No Rights as Stockholder. This Warrant shall not entitle
                  ------------------------
the Holder to any rights as a stockholder of the Company.

         IN WITNESS WHEREOF, Zoltek Companies, Inc. has caused this Warrant
to be executed in its corporate name by one of its officers thereunto duly
authorized, all as of the day and year first above written.

                              ZOLTEK COMPANIES, INC.


                              By: /s/ Zsolt Rumy
                              Name: Zsolt Rumy
                              Title: President






                                     7

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                             SUBSCRIPTION NOTICE

                  (To be executed upon exercise of Warrant)

To:
         -----------------------------------

         The undersigned hereby irrevocably elects to exercise the right
of purchase represented by the attached Warrant for, and to purchase
thereunder, ________ shares of Common Stock, as provided for therein, and
tenders herewith payment of the Exercise Price in full in the form of cash,
certified or bank cashier's check or wire transfer.

         Please issue a certificate or certificates for such shares of
Common Stock in the following name or names and denominations:






         If said number of shares shall not equal all the shares issuable
upon exercise of the attached Warrant, a new Warrant is to be issued in the
name of the undersigned for the balance remaining of such shares less any
fraction of a share paid in cash.


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




                                     ---------------------------------------
                                     NOTE:  The above signature should
                                            correspond exactly with the name
                                            on the face of the attached
                                            Warrant or with the name of the
                                            assignee appearing in the
                                            following assignment form.



                                     8

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                                 ASSIGNMENT

                 (To be executed upon assignment of Warrant)


         For value received, ________________________, hereby sells, assigns
and transfers unto ________________________ the attached Warrant, together
with all right, title and interest therein, and does hereby irrevocably
constitute and appoint ________________________ attorney to transfer said
Warrant on the books of ______________________, a ______________________
corporation, with full power of substitution in the premises.



                                     ---------------------------------------
                                     Note:  The above signature should
                                            correspond exactly with the name
                                            on the face of the attached
                                            Warrant.


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




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