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                                                                     EXHIBIT 4.1


                              NEOTHERAPEUTICS, INC.

                                     WARRANT

DATED:  MAY 18, 2001                                              NUMBER NEOT002


     NeoTherapeutics, Inc., a Delaware corporation (the "Company"), hereby
certifies that, for value received, MONTROSE INVESTMENTS LTD., or its registered
assigns ("Holder"), is entitled, subject to the terms set forth below, to
purchase from the Company up to a total of 180,000 shares of Common Stock, $.001
par value per share (the "Common Stock"), of the Company (each such share, a
"Warrant Share" and all such shares, the "Warrant Shares") at an exercise price
equal to $6.00 per share (as adjusted from time to time as provided in Section
7, the "Exercise Price"), at any time and from time to time from and after
December 31, 2002 (or such earlier date as may be determined pursuant to Section
3(a) below) and through and including May 17, 2006 (the "Expiration Date"), and
subject to the following terms and conditions:

     1.   Registration of Warrant. The Company shall register this Warrant upon
records to be maintained by the Company for that purpose (the "Warrant
Register"), in the name of the record Holder hereof from time to time. The
Company may deem and treat the registered Holder of this Warrant as the absolute
owner hereof for the purpose of any exercise hereof or any distribution to the
Holder, and for all other purposes, and the Company shall not be affected by
notice to the contrary.

     2.   Registration of Transfers and Exchanges.

          (a)  This Warrant may only be transferred pursuant to an effective
registration statement under the Securities Act, to the Company or pursuant to
an available exemption from or in a transaction not subject to the registration
requirements of the Securities Act. In connection with any transfer of this
Warrant other than pursuant to an effective registration statement or to the
Company, the Company may require the transferor thereof to provide to the
Company an opinion of counsel selected by the transferor, the form and substance
of which opinion shall be reasonably satisfactory to the Company, to the effect
that such transfer does not require registration of such transferred securities
under the Securities Act.

          (b)  The Company shall register the transfer of any portion of this
Warrant in conformance with Section 2(a) in the Warrant Register, upon surrender
of this Warrant, with the Form of Assignment attached hereto duly completed and
signed, to the Company at the office specified in or pursuant to Section 10.
Upon any such registration or transfer, a new warrant to purchase Common Stock,
in substantially the form of this Warrant (any such new warrant, a "New
Warrant"), evidencing the portion of this Warrant so transferred shall be issued
to the transferee and a New Warrant evidencing the remaining portion of this
Warrant not so transferred, if any, shall be issued to the transferring Holder.
The acceptance of the New Warrant by the transferee thereof shall be deemed the
acceptance of such transferee of all of the rights and obligations of a holder
of this Warrant.

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          (c)  This Warrant is exchangeable, upon the surrender hereof by the
Holder to the office of the Company specified in or pursuant to Section 10 for
one or more New Warrants, evidencing in the aggregate the right to purchase the
number of Warrant Shares which may then be purchased hereunder.

     3.   Duration and Exercise of Warrant.

          (a)  This Warrant shall be exercisable by the then registered Holder
on any business day before 5:00 P.M., California time, at any time and from time
to time on or after the earlier to occur of: (i) December 31, 2002; (ii) a date
specified in a written notice from the registered Holder, which date shall be
not less than 65 days after the date of such written notice; and (iii) one
business day prior to the occurence of any event specified in Section 7(b) or
7(f)(i)-(v) below, to and including the Expiration Date. At 5:00 P.M.,
California time on the Expiration Date, the portion of this Warrant not
exercised prior thereto shall be and become void and of no value.

          (b)  Upon surrender of this Warrant, with the Form of Election to
Purchase attached hereto duly completed and signed, to the Company at its
address for notice set forth in Section 10 and upon payment of the Exercise
Price multiplied by the number of Warrant Shares that the Holder intends to
purchase hereunder, in the manner provided hereunder, all as specified by the
Holder in the Form of Election to Purchase, the Company shall promptly issue or
cause to be issued and cause to be delivered to or upon the written order of the
Holder and in such name or names as the Holder may designate, a certificate for
the Warrant Shares issuable upon such exercise. Any person so designated by the
Holder to receive Warrant Shares shall be deemed to have become holder of record
of such Warrant Shares as of the Date of Exercise of this Warrant.

          A "Date of Exercise" means the date on which the Company shall have
received (i) this Warrant (or any New Warrant, as applicable), with the Form of
Election to Purchase attached hereto (or attached to such New Warrant)
appropriately completed and duly signed, and (ii) payment of the Exercise Price
for the number of Warrant Shares so indicated by the holder hereof to be
purchased.

          (c)  This Warrant shall be exercisable, either in its entirety or,
from time to time, for a portion of the number of Warrant Shares. If less than
all of the Warrant Shares which may be purchased under this Warrant are
exercised at any time, the Company shall issue or cause to be issued, at its
expense, a New Warrant evidencing the right to purchase the remaining number of
Warrant Shares for which no exercise has been evidenced by this Warrant.

          (d)  Prior to the exercise of this Warrant, the Holder shall not be
entitled to any rights as a stockholder of the Company with respect to the
Warrant Shares, including (without limitation) the right to vote such shares,
receive dividends or other distributions thereon or be notified of stockholder
meetings (except as otherwise set forth in Section 7(f) herein).

     4.   Payment of Taxes. The Company will pay any documentary stamp taxes
attributable to the issuance of Warrant Shares 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 Shares or Warrants in a

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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 Shares upon exercise hereof.

     5.   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, but only upon receipt of evidence reasonably
satisfactory to the Company of such loss, theft or destruction and indemnity, if
requested, satisfactory to it. Applicants for a New Warrant under such
circumstances shall also comply with such other reasonable regulations and
procedures and pay such other reasonable charges as the Company may prescribe.

     6.   Reservation of Warrant Shares. The Company covenants that it will at
all times reserve and keep available out of the aggregate of its authorized but
unissued Common Stock, solely for the purpose of enabling it to issue Warrant
Shares upon exercise of this Warrant as herein provided, the number of Warrant
Shares which are then issuable and deliverable upon the exercise of this entire
Warrant, free from preemptive rights or any other actual contingent purchase
rights of persons other than the Holder. The Company covenants that all Warrant
Shares that shall be so issuable and deliverable shall, upon issuance and the
payment of the applicable Exercise Price in accordance with the terms hereof, be
duly authorized, validly issued and fully paid and nonassessable.

     7.   Certain Adjustments. The Exercise Price and number of Warrant Shares
issuable upon exercise of this Warrant are subject to adjustment from time to
time as set forth in this Section 7. Upon each such adjustment of the Exercise
Price pursuant to this Section 7, the Holder shall thereafter prior to the
Expiration Date be entitled to purchase, at the Exercise Price resulting from
such adjustment, the number of Warrant Shares obtained by multiplying the
Exercise Price in effect immediately prior to such adjustment by the number of
Warrant Shares issuable upon exercise of this Warrant immediately prior to such
adjustment and dividing the product thereof by the Exercise Price resulting from
such adjustment.

          (a)  If the Company, at any time while this Warrant is outstanding,
(i) shall pay a stock dividend (except scheduled dividends paid on outstanding
preferred stock which contain a stated dividend rate) or otherwise make a
distribution or distributions on shares of its Common Stock payable in shares of
Common Stock, (ii) subdivide outstanding shares of Common Stock into a larger
number of shares, or (iii) combine outstanding shares of Common Stock into a
smaller number of shares, the Exercise Price shall be multiplied by a fraction
of which the numerator shall be the number of shares of Common Stock (excluding
treasury shares, if any) outstanding before such event and the denominator shall
be the number of shares of Common Stock (excluding treasury shares, if any)
outstanding after such event. Any adjustment made pursuant to this Section shall
become effective immediately after the record date for the determination of
stockholders entitled to receive such dividend or distribution and shall become
effective immediately after the effective date in the case of a subdivision or
combination, and shall apply to successive subdivisions and combinations.

          (b)  In case of any reclassification of the Common Stock, any
consolidation or merger of the Company with or into another person, the sale or
transfer of all or substantially all

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of the assets of the Company or any compulsory share exchange pursuant to which
the Common Stock is converted into other securities, cash or property, then the
Holder shall have the right thereafter to exercise this Warrant only into the
shares of stock and other securities and property receivable upon or deemed to
be held by holders of Common Stock following such reclassification,
consolidation, merger, sale, transfer or share exchange, and the Holder shall be
entitled upon such event to receive such amount of securities or property equal
to the amount of Warrant Shares such Holder would have been entitled to had such
Holder exercised this Warrant immediately prior to such reclassification,
consolidation, merger, sale, transfer or share exchange. The terms of any such
consolidation, merger, sale, transfer or share exchange shall include such terms
so as to continue to give to the Holder the right to receive the securities or
property set forth in this Section 7(b) upon any exercise following any such
reclassification, consolidation, merger, sale, transfer or share exchange.

          (c)  If the Company, at any time while this Warrant is outstanding,
shall distribute to all holders of Common Stock (and not to holders of this
Warrant) evidences of its indebtedness or assets or rights or warrants to
subscribe for or purchase any security (excluding those referred to in Sections
7(a), and (b)), other than as part of its dissolution or liquidation or the
winding up of its affairs, then in each such case the Exercise Price shall be
determined by multiplying the Exercise Price in effect immediately prior to the
record date fixed for determination of stockholders entitled to receive such
distribution by a fraction of which the denominator shall be the fair market
value of a share of Common Stock determined as of the record date mentioned
above, and of which the numerator shall be the fair market value of a share of
Common Stock determined as of such record date less the fair market value at
such record date of the portion of such assets or evidence of indebtedness so
distributed applicable to one outstanding share of Common Stock as determined by
the Company's independent certified public accountants that regularly examines
the financial statements of the Company (an "Appraiser").

          (d)  For the purposes of this Section 7, the following clauses shall
also be applicable:

               (i)  Record Date. In case the Company shall take a record of the
                    holders of its Common Stock for the purpose of entitling
                    them (A) to receive a dividend or other distribution payable
                    in Common Stock or in securities convertible or exchangeable
                    into shares of Common Stock, or (B) to subscribe for or
                    purchase Common Stock or securities convertible or
                    exchangeable into shares of Common Stock, then such record
                    date shall be deemed to be the date of the issue or sale of
                    the shares of Common Stock deemed to have been issued or
                    sold upon the declaration of such dividend or the making of
                    such other distribution or the date of the granting of such
                    right of subscription or purchase, as the case may be.

               (ii) Treasury Shares. The number of shares of Common Stock
                    outstanding at any given time shall not include shares owned
                    or held by or for the account of the Company, and the
                    disposition of

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                    any such shares shall be considered an issue or sale of
                    Common Stock.

          (e)  All calculations under this Section 7 shall be made to the
nearest cent or the nearest 1/100th of a share, as the case may be.

          (f)  If:

               (i)   the Company shall declare a dividend (or any other
                     distribution) on its Common Stock; or

               (ii)  the Company shall declare a special nonrecurring cash
                     dividend on or a redemption of its Common Stock; or

               (iii) the Company shall authorize the granting to all holders of
                     the Common Stock rights or warrants to subscribe for or
                     purchase any shares of capital stock of any class or of any
                     rights; or

               (iv)  the approval of any stockholders of the Company shall be
                     required in connection with any reclassification of the
                     Common Stock of the Company, any consolidation or merger to
                     which the Company is a party, any sale or transfer of all
                     or substantially all of the assets of the Company, or any
                     compulsory share exchange whereby the Common Stock is
                     converted into other securities, cash or property; or

               (v)   the Company shall authorize the voluntary dissolution,
                     liquidation or winding up of the affairs of the Company,

then the Company shall cause to be mailed to each Holder at their last addresses
as they shall appear upon the Warrant Register, at least 30 calendar days prior
to the applicable record or effective date hereinafter specified, a notice
stating (x) the date on which a record is to be taken for the purpose of such
dividend, distribution, redemption, rights or warrants, or if a record is not to
be taken, the date as of which the holders of Common Stock of record to be
entitled to such dividend, distributions, redemption, rights or warrants are to
be determined or (y) the date on which such reclassification, consolidation,
merger, sale, transfer or share exchange is expected to become effective or
close, and the date as of which it is expected that holders of Common Stock of
record shall be entitled to exchange their shares of Common Stock for
securities, cash or other property deliverable upon such reclassification,
consolidation, merger, sale, transfer, share exchange, dissolution, liquidation
or winding up; provided, however, that the failure to mail such notice or any
defect therein or in the mailing thereof shall not affect the validity of the
corporate action required to be specified in such notice.

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     8.   Payment of Exercise Price. The Holder shall pay the Exercise Price in
one of the following manners:

          (a)  Cash Exercise. The Holder may deliver immediately available funds
by certified check or bank draft payable to the order of the Company or by wire
transfer to an account designated by the Company; or

          (b)  Cashless Exercise. The Holder may surrender this Warrant to the
Company together with a notice of cashless exercise, in which event the Company
shall issue to the Holder the number of Warrant Shares determined as follows:

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

     where:

     X = the number of Warrant Shares to be issued to the Holder.

     Y = the number of Warrant Shares with respect to which this Warrant is
being exercised.

     A = the average of the closing sale prices of the Common Stock for the five
(5) trading days immediately prior to (but not including) the Date of Exercise.

     B = the Exercise Price.

     For purposes of Rule 144 promulgated under the Securities Act of 1933, as
amended, it is intended, understood and acknowledged that the Warrant Shares
issued in a cashless exercise transaction shall be deemed to have been acquired
by the Holder, and the holding period for the Warrant Shares shall be deemed to
have been commenced, on the issue date.

     9.   Fractional Shares. The Company shall not be required to issue or cause
to be issued fractional Warrant Shares on the exercise of this Warrant. The
number of full Warrant Shares which shall be issuable upon the exercise of this
Warrant shall be computed on the basis of the aggregate number of Warrant Shares
purchasable on exercise of this Warrant so presented. If any fraction of a
Warrant Share would, except for the provisions of this Section 9, be issuable on
the exercise of this Warrant, the Company shall pay an amount in cash equal to
the Exercise Price multiplied by such fraction.

     10.  Certain Exercise Restrictions.

          (a)  A Holder may not exercise this Warrant to the extent such
exercise would result in the Holder, together with any affiliate thereof,
beneficially owning (as determined in accordance with Section 13(d) of the
Securities Exchange Act of 1934, as amended and the rules promulgated
thereunder) in excess of 4.999% of the then issued and outstanding shares of
Common Stock, including shares of Common Stock issuable upon such exercise and
held by such Holder after application of this Section. Since the Holder will not
be obligated to report to the Company the number of shares of Common Stock it
may hold at the time of an exercise hereunder, unless the exercise at issue
would result in the issuance of shares of Common Stock in excess of 4.999% of
the then outstanding shares of Common Stock without regard to any other shares
which may be beneficially owned by the Holder or an affiliate thereof, the
Holder shall

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have the authority and obligation to determine whether the restriction contained
in this Section will limit any particular exercise hereunder and to the extent
that the Holder determines that the limitation contained in this Section
applies, the determination of which portion of this Warrant is exercisable shall
be the responsibility and obligation of the Holder. If the Holder has delivered
a Form of Election to Purchase for a number of Warrant Shares that, without
regard to any other shares that the Holder or its affiliates may beneficially
own, would result in the issuance in excess of the permitted amount hereunder,
the Company shall notify the Holder of this fact and shall honor the exercise
for the maximum portion of this Warrant permitted to be exercised on such Date
of Exercise in accordance with the periods described herein and, at the option
of the Holder, either keep the portion of the Warrant tendered for exercise in
excess of the permitted amount hereunder for future exercises or return such
excess portion of the Warrant to the Holder. The provisions of this Section may
be waived by a Holder (but only as to itself and not to any other Holder) upon
not less than 61 days prior notice to the Company. Other Holders shall be
unaffected by any such waiver.

          (b)  A Holder may not exercise this Warrant to the extent such
exercise would result in the Holder, together with any affiliate thereof,
beneficially owning (as determined in accordance with Section 13(d) of the
Securities Exchange Act of 1934, as amended and the rules promulgated
thereunder) in excess of 9.999% of the then issued and outstanding shares of
Common Stock, including shares of Common Stock issuable upon such exercise and
held by such Holder after application of this Section. Since the Holder will not
be obligated to report to the Company the number of shares of Common Stock it
may hold at the time of an exercise hereunder, unless the exercise at issue
would result in the issuance of shares of Common Stock in excess of 9.999% of
the then outstanding shares of Common Stock without regard to any other shares
which may be beneficially owned by the Holder or an affiliate thereof, the
Holder shall have the authority and obligation to determine whether the
restriction contained in this Section will limit any particular exercise
hereunder and to the extent that the Holder determines that the limitation
contained in this Section applies, the determination of which portion of this
Warrant is exercisable shall be the responsibility and obligation of the Holder.
If the Holder has delivered a Form of Election to Purchase for a number of
Warrant Shares that, without regard to any other shares that the Holder or its
affiliates may beneficially own, would result in the issuance in excess of the
permitted amount hereunder, the Company shall notify the Holder of this fact and
shall honor the exercise for the maximum portion of this Warrant permitted to be
exercised on such Date of Exercise in accordance with the periods described
herein and, at the option of the Holder, either keep the portion of the Warrant
tendered for exercise in excess of the permitted amount hereunder for future
exercises or return such excess portion of the Warrant to the Holder. The
provisions of this Section may be waived by a Holder (but only as to itself and
not to any other Holder) upon not less than 61 days prior notice to the Company.
Other Holders shall be unaffected by any such waiver.

     11.  Notices. Any and all notices or other communications or deliveries
hereunder shall be in writing and shall be deemed given and effective on the
earliest of (i) the date of transmission, if such notice or communication is
delivered via facsimile at the facsimile telephone number specified in this
Section prior to 5:00 p.m. (California time) on a business day, (ii) the
business day after the date of transmission, if such notice or communication is
delivered via facsimile at the facsimile telephone number specified in this
Section later than 5:00 p.m. (California time) on any date and earlier than
11:59 p.m. (California time) on such date, (iii) the business day following the
date of mailing, if sent by nationally recognized overnight courier service, or
(iv) upon actual receipt by the party to whom such notice is required to be
given. The

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addresses for such communications shall be: (i) if to the Company, to 157
Technology Drive, Irvine, CA 92618, Attention: Chief Financial Officer, or to
facsimile no. (949) 788-6706, or (ii) if to the Holder, to the Holder at the
address or facsimile number appearing on the Warrant Register or such other
address or facsimile number as the Holder may provide to the Company in
accordance with this Section 11.

     12.  Warrant Agent. The Company shall serve as warrant agent under this
Warrant. The Company may appoint a new warrant agent upon notice to the Holder
in accordance with Section 11. Any corporation into which the Company may be
merged or any corporation resulting from any consolidation to which the Company
shall be a party or any corporation to which the Company transfers substantially
all of its corporate assets shall be a successor warrant agent under this
Warrant without any further act. Any such successor warrant agent shall promptly
cause notice of its succession as warrant agent to be mailed (by first class
mail, postage prepaid) to the Holder at the Holder's last address as shown on
the Warrant Register.

     13.  Miscellaneous.

          (a)  This Warrant shall be binding on and inure to the benefit of the
parties hereto and their respective successors and permitted assigns. This
Warrant may be amended only in writing signed by the Company and the Holder and
their successors and assigns.

          (b)  Subject to Section 13(a), above, nothing in this Warrant shall be
construed to give to any person or corporation other than the Company and the
Holder any legal or equitable right, remedy or cause under this Warrant. This
Warrant shall inure to the sole and exclusive benefit of the Company and the
Holder.

          (c)  This Warrant shall be governed by and construed and enforced in
accordance with the internal laws of the State of New York without regard to the
principles of conflicts of law thereof. The Company and the Holder hereby
irrevocably submit to the exclusive jurisdiction of the state and federal courts
sitting in City of New York, borough of Manhattan, for the adjudication of any
dispute hereunder or in connection herewith or with any transaction contemplated
hereby or discussed herein, and hereby irrevocably waives, and agrees not to
assert in any suit, action or proceeding, any claim that it is not personally
subject to the jurisdiction of any such court, or that such suit, action or
proceeding is improper. Each of the Company and the Holder hereby irrevocably
waives personal service of process and consents to process being served in any
such suit, action or proceeding by receiving a copy thereof sent to the Company
at the address in effect for notices to it under this instrument and agrees that
such service shall constitute good and sufficient service of process and notice
thereof. Nothing contained herein shall be deemed to limit in any way any right
to serve process in any manner permitted by law.

          (d)  The headings herein are for convenience only, do not constitute a
part of this Warrant and shall not be deemed to limit or affect any of the
provisions hereof.

          (e)  In case any one or more of the provisions of this Warrant shall
be invalid or unenforceable in any respect, the validity and enforceability of
the remaining terms and provisions of this Warrant shall not in any way be
affected or impaired thereby and the parties

<PAGE>   9

will attempt in good faith to agree upon a valid and enforceable provision which
shall be a commercially reasonable substitute therefor, and upon so agreeing,
shall incorporate such substitute provision in this Warrant.

     IN WITNESS WHEREOF, the Company has caused this Warrant to be duly executed
by its authorized officer as of the date first indicated above.


                                              NEOTHERAPEUTICS, INC.


                                              By: /s/ Samuel Gulko
                                                  ------------------------------
                                              Name:  Samuel Gulko
                                              Title: Senior Vice President,
                                                     Finance and Chief Financial
                                                     Officer

<PAGE>   10

                          FORM OF ELECTION TO PURCHASE


(To be executed by the Holder to exercise the right to purchase shares of Common
Stock under the foregoing Warrant)


To NeoTherapeutics, Inc.:

     In accordance with the Warrant enclosed with this Form of Election to
Purchase, the undersigned hereby irrevocably elects to purchase __________
shares of Common Stock ("Common Stock"), $.001 par value per share, of
NeoTherapeutics, Inc. encloses herewith $__________ in cash, certified or
official bank check or checks, which sum represents the aggregate Exercise Price
(as defined in the Warrant) for the number of shares of Common Stock to which
this Form of Election to Purchase relates, together with any applicable taxes
payable by the undersigned pursuant to the Warrant.

     The undersigned requests that certificates for the shares of Common Stock
issuable upon this exercise be issued in the name of

                                               PLEASE INSERT SOCIAL SECURITY OR
                                               TAX IDENTIFICATION NUMBER

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


--------------------------------------------------------------------------------
                         (Please print name and address)

     If the number of shares of Common Stock issuable upon this exercise shall
not be all of the shares of Common Stock which the undersigned is entitled to
purchase in accordance with the enclosed Warrant, the undersigned requests that
a New Warrant (as defined in the Warrant) evidencing the right to purchase the
shares of Common Stock not issuable pursuant to the exercise evidenced hereby be
issued in the name of and delivered to:


--------------------------------------------------------------------------------
                         (Please print name and address)

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

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

<PAGE>   11

Dated:                       ,                    Name of Holder:
       ----------------------  -----------
                                                  (Print)
                                                         -----------------------

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

                                                  (Name:)
                                                  (Title:)
                                                  (Signature must conform in all
                                                  respects to name of holder as
                                                  specified on the face of the
                                                  Warrant)

<PAGE>   12

                               FORM OF ASSIGNMENT

           [To be completed and signed only upon transfer of Warrant]

     FOR VALUE RECEIVED, the undersigned hereby sells, assigns and transfers
unto ____________________ the right represented by the Warrant enclosed with
this Form of Assignment to purchase __________ shares of Common Stock of
NeoTherapeutics, Inc. to which the Warrant relates and appoints
____________________ attorney to transfer said right on the books of
NeoTherapeutics, Inc. with full power of substitution in the premises.


Dated:                       ,
       ----------------------  -----------

                                                --------------------------------
                                                (Signature must conform in all
                                                respects to name of holder as
                                                specified on the face of the
                                                Warrant)


                                                --------------------------------
                                                Address of Transferee

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


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

In the presence of:


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