<PAGE>
                                                                   EXHIBIT 10.37

                                                                       No. W-001

THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR INVESTMENT
AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 OR ANY STATE
SECURITIES LAWS.  SUCH SECURITIES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE
OF SUCH REGISTRATION OR AN EXEMPTION THEREFROM UNDER SAID ACT AND ANY APPLICABLE
STATE SECURITIES LAWS.

                       WARRANT TO PURCHASE NEW SECURITIES
                    OF HEALTHCARE TRANSACTION SYSTEMS, INC.
                           (Void after April 1, 2002)



     This certifies that Zilkha Venture Partners, L.P. or its assigns (the
"Holder"), for value received, is entitled to purchase from HEALTHCARE
TRANSACTION SYSTEMS, INC., a Delaware corporation (the "Company"), having a
place of business at 75 Willow Road, Menlo Park, California 94025, that number
of fully paid and nonassessable shares of the capital stock of the Company as
provided for herein.  This Warrant is one of a series of like Warrants (the
"Warrants") to be issued pursuant to the terms of that certain Note and Warrant
Purchase Agreement (the "Agreement") dated as of April 2, 1999 among the Company
and the other parties named therein.  This Warrant is issued in conjunction with
a note (the "Note") issued by the Company pursuant to the Agreement on the date
hereof in the principal amount of $150,000.00 (the "Loan Amount").

     During the Exercise Period (defined below) the Holder may purchase from the
Company that number of fully paid and nonassessable shares of the capital stock
of the Company ("New Securities") as shall equal (a) 17.6% multiplied by the
Loan Amount, divided by (b) the New Security Price (as defined below).  The New
Securities shall be of the same class and series and have the same rights,
preferences and privileges as (i) the Series B Shares (as defined in the Note),
in the event the Note converts pursuant to Section 3 thereof or (ii) the Series
A Shares (as defined in the Note), in the event the Note converts pursuant to
either of Sections 4 or 5 thereof, or in the event the Note becomes due and
payable pursuant to Section 6 thereof.  Unless otherwise defined in this
Warrant, capitalized terms used in this Warrant shall have the meaning indicated
in the Agreement and the Note.  This Warrant shall be exercisable only during
the period (the "Exercise Period") commencing on the date that the Note shall be
either converted or repaid pursuant to Sections 3, 4, 5 or 6 of the Note and
ending at 5:00 p.m. (Pacific time) on the earlier of (i) the closing of the
initial public offering of the Company's Securities pursuant to a registration
statement under the Securities Act of 1933, as amended (the "Initial Public
Offering"); or (ii) three (3) years from the date of this Warrant, such earlier
day being referred to herein as the "Expiration Date."  To exercise this
Warrant, the Holder shall surrender to the Company at its principal office (or
at such other location as the Company may advise the Holder in writing) the
original Warrant, properly endorsed with the Form of Subscription attached
hereto duly filled in and signed, and shall pay in cash or by check the
aggregate New Security Price (as
<PAGE>

defined below), unless exercised as provided in Section 1.2 of this Warrant, for
the number of New Securities for which this Warrant is being exercised,
determined in accordance with the provisions hereof. The Company shall deliver
notice of any Initial Public Offering to the Holder at least 30 days prior to
the closing thereof. The New Security Price and the number of New Securities
purchasable hereunder are subject to adjustment as provided in Section 3 of this
Warrant.

     This Warrant is subject to the following terms and conditions:

           1.1  EXERCISE; ISSUANCE OF CERTIFICATES; PAYMENT FOR NEW SECURITIES.
This Warrant is exercisable at the option of the holder of record hereof only
during the Exercise Period. The exercise price for this Warrant (the "New
Security Price") shall equal (i) the Financing Price (as defined in the Note),
in the event the Note converts pursuant to Section 3 thereof, (ii) the Bridge A
Price (as defined in the Note), in the event the Note converts pursuant to
Sections 4 or 5 thereof, or (iii) the Bridge A Price (as defined in the Note),
in the event the Note becomes due and payable pursuant to Section 6 thereof. The
Company agrees that the New Securities purchased under this Warrant shall be and
are deemed to be issued to the Holder hereof as the record owner of such New
Securities as of the close of business on the date on which this Warrant shall
have been surrendered, properly endorsed, the completed, executed Form of
Subscription delivered, and payment made for such New Securities. Certificates
for the New Securities so purchased, together with any other securities or
property to which the Holder hereof is entitled upon such exercise, shall be
delivered to the Holder hereof by the Company at the Company's expense within a
reasonable time after the rights represented by this Warrant have been so
exercised. In case of a purchase of less than all the New Securities which may
be purchased under this new Warrant, the Company shall cancel this Warrant and
execute and deliver a new Warrant or Warrants of like tenor for the balance of
the New Securities purchasable under the Warrant surrendered upon such purchase
to the Holder hereof within a reasonable time.

           1.2  Net Issue Exercise.  Notwithstanding any provisions herein to
the contrary, if the fair market value of one share of the Company's New
Securities is greater than the New Security Price (at the date of calculation as
set forth below), in lieu of exercising this Warrant for cash, the Holder may
elect to receive shares equal to the value (as determined below) of this Warrant
(or the portion thereof being canceled) by surrender of this Warrant at the
principal office of the Company together with the properly endorsed Form of
Subscription and notice of such election in which event the Company shall issue
to the Holder a number of shares of New Securities computed using the following
formula:

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

         Where  X =  the number of shares of New Securities to be issued to
                     the Holder

                Y =  the number of shares of New Securities purchasable under
                     the Warrant or, if only a portion of the Warrant is being
                     exercised, the portion of the Warrant being canceled (at
                     the date of such calculation)

                                       2.
<PAGE>

                A =  the fair market value of one share of the Company's New
                     Securities (at the date of such calculation)

                B =  New Security Price (as adjusted to the date of such
                     calculation)

For purposes of the above calculation, fair market value of one share of New
Securities shall be determined by the Company's Board of Directors in good
faith; provided, however, that in the event the Company makes an initial public
offering of its Common Stock the fair market value per share shall be the
product of (i) the per share offering price to the public of the Company's
initial public offering, and (ii) the number of shares of Common Stock into
which each share of New Securities is convertible at the time of such exercise.

     2.   NEW SECURITIES TO BE FULLY PAID; RESERVATION OF NEW SECURITIES. The
Company covenants and agrees that all New Securities which may be issued upon
the exercise of the rights represented by this Warrant will, upon issuance, be
duly authorized, validly issued, fully paid and nonassessable and free from all
preemptive rights of any shareholder and free of all taxes, liens and charges
with respect to the issue thereof. The Company further covenants and agrees
that, during the Exercise Period, the Company will at all times have authorized
and reserved, for the purpose of issue or transfer upon exercise of the
subscription rights evidenced by this Warrant, a sufficient number of New
Securities, or other securities and property, when and as required to provide
for the exercise of the rights represented by this Warrant. The Company will
take all such action as may be necessary to assure that such New Securities may
be issued as provided herein without violation of any applicable law or
regulation, or of any requirements of any domestic securities exchange upon
which the New Securities may be listed; provided, however, that the Company
shall not be required to effect a registration under Federal or State securities
laws with respect to such exercise.

     3.  ADJUSTMENT OF NEW SECURITY PRICE AND NUMBER OF NEW SECURITIES. The New
Security Price and the number of New Securities purchasable upon the exercise of
this Warrant shall be subject to adjustment from time to time upon the
occurrence of certain events described in this Section 3.

           3.1  Subdivision or Combination of New Securities.

                (a)  If at any time or from time to time after the date of this
Warrant the Company shall subdivide its outstanding New Securities, the New
Security Price in effect immediately prior to such subdivision shall be
proportionately reduced, and conversely, in case the New Securities of the
Company shall be combined into a smaller number of New Securities, the New
Security Price in effect immediately prior to such combination shall be
proportionately increased.

                (b)  Upon each adjustment of the New Security Price as provided
in Section 3.1(a) above, the Holder shall thereafter be entitled to purchase, at
the New Security Price resulting from such adjustment, the number of New
Securities (calculated to the nearest whole Security) obtained by multiplying
the New Security Price in effect immediately prior to such adjustment by the
number of New Securities purchasable pursuant hereto immediately prior

                                       3.
<PAGE>

to such adjustment and dividing the product thereof by the New Security Price
resulting from such adjustment.

           3.2  Reorganization, Reclassification, Consolidation, Merger or Sale.
Subject to the first paragraph above, if any recapitalization, reclassification
or reorganization of the capital stock of the Company, or any consolidation or
merger of the Company with another corporation, or the sale of all or
substantially all of its assets or other transaction shall be effected in such a
way that holders of New Securities shall be entitled to receive stock,
securities, or other assets or property (a "Reorganization") then, as a
condition of such Reorganization, lawful and adequate provisions shall be made
by the Company whereby the Holder hereof shall thereafter have the right to
purchase and receive (in lieu of the New Securities of the Company immediately
theretofore purchasable and receivable upon the exercise of the rights
represented hereby) such shares of securities or other assets or property as may
be issued or payable with respect to or in exchange for a number of outstanding
New Securities equal to the number of New Securities immediately theretofore
purchasable and receivable upon the exercise of the rights represented hereby.
In the event of any Reorganization, appropriate provision shall be made by the
Company with respect to the rights and interests of the Holder of this Warrant
to the end that the provisions hereof (including, without limitation, provisions
for adjustments of the New Security Price and of the number of shares
purchasable and receivable upon the exercise of this Warrant) shall thereafter
be applicable, in relation to any shares of securities or assets thereafter
deliverable upon the exercise hereof. The Company will not effect any such
consolidation, merger or sale unless, prior to the consummation thereof, the
successor corporation (if other than the Company) resulting from such
consolidation or the corporation purchasing such assets shall assume by written
instrument reasonably satisfactory in form and substance to the Holders of a
majority of the warrants to purchase New Securities then outstanding, executed
and mailed or delivered to the registered Holder hereof at the last address of
such Holder appearing on the books of the Company, the obligation to deliver to
such Holder such shares of stock, securities or assets as, in accordance with
the foregoing provisions, such Holder may be entitled to purchase.

           3.3  Certain Events.  If any change in the outstanding New Securities
of the Company or any other event occurs as to which the other provisions of
this Section 3 are not strictly applicable or if strictly applicable would not
fairly protect the purchase rights of the Holder of the Warrant in accordance
with such provisions, then the Board of Directors of the Company shall make an
adjustment in the number and class of New Securities available under the
Warrant, the New Security Price or the application of such provisions, so as to
protect such purchase rights as aforesaid. The adjustment shall be such as will
give the Holder of the Warrant upon exercise for the same aggregate New Security
Price the total number, class and kind of New Securities as he would have owned
had the Warrant been exercised prior to the event and had he continued to hold
such shares until after the event requiring adjustment.

           3.4  Notices of Change.

                (a)  Immediately upon any adjustment in the number or class of
New Securities subject to this Warrant and of the New Security Price, the
Company shall give written

                                       4.
<PAGE>

notice thereof to the Holder, setting forth in reasonable detail and certifying
the calculation of such adjustment.

                (b)  The Company shall give written notice to the Holder at
least 10 business days prior to the date on which the Company closes its books
or takes a record for determining rights to receive any dividends or
distributions. (c) The Company shall also give written notice to the Holder at
least 30 business days prior to the date on which a Reorganization shall take
place.

     4.  ISSUE TAX.  The issuance of certificates for New Securities upon the
exercise of the Warrant shall be made without charge to the Holder of the
Warrant for any issue tax (other than any applicable income taxes) in respect
thereof; 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 issuance and
delivery of any certificate in a name other than that of the then Holder of the
Warrant being exercised.

     5.  CLOSING OF BOOKS.  The Company will at no time close its transfer books
against the transfer of any warrant or of any New Securities issued or issuable
upon the exercise of any warrant in any manner which interferes with the timely
exercise of this Warrant.

     6.  NO VOTING OR DIVIDEND RIGHTS; LIMITATION OF LIABILITY.  Nothing
contained in this Warrant shall be construed as conferring upon the Holder
hereof the right to vote or to consent or to receive notice as a member of the
Company or any other matters or any rights whatsoever as a member of the
Company. In the event the Company shall declare a distribution payable in
securities of other persons, evidences of indebtedness issued by the Company or
other persons, assets (excluding cash dividends) or options or rights, then, in
each such case for the purpose of this Section 6, upon exercise of this Warrant,
the Holder hereof shall be entitled to a proportionate share of any such
distribution as though such Holder was the holder of the number of shares of New
Securities of the Company into which this Warrant may be exercised as of the
record date fixed for the determination of the holders of New Securities of the
Company entitled to receive such distribution.. No provisions hereof, in the
absence of affirmative action by the holder to purchase New Securities, and no
mere enumeration herein of the rights or privileges of the holder hereof, shall
give rise to any liability of such Holder for the New Security Price or as a
shareholder of the Company, whether such liability is asserted by the Company or
by its creditors.

     7.  TRANSFERABILITY OF WARRANT.  This Warrant may not be transferred or
assigned in whole or in part.

     8.  RIGHTS AND OBLIGATIONS SURVIVE EXERCISE OF WARRANT.  The rights and
obligations of the Company, of the holder of this Warrant and of the holder of
New Securities issued upon exercise of this Warrant, shall survive the exercise
of this Warrant.

                                       5.
<PAGE>

     9.  MODIFICATION AND WAIVER.  This Warrant and any provision hereof may be
changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

     10. NOTICES.  Any notice, request or other document required or permitted
to be given or delivered to the holder hereof or the Company shall be delivered
or shall be sent by certified mail, postage prepaid, to each such holder at its
address as shown on the books of the Company or to the Company at the address
indicated therefor in the first paragraph of this Warrant or such other address
as either may from time to time provide to the other.

     11.  BINDING EFFECT ON SUCCESSORS.  This Warrant shall be binding upon any
corporation succeeding the Company by merger, consolidation or acquisition of
all or substantially all of the Company's assets.  All of the obligations of the
Company relating to the New Securities issuable upon the exercise of this
Warrant shall survive the exercise and termination of this Warrant.  All of the
covenants and agreements of the Company shall inure to the benefit of the
successors and assigns of the holder hereof.

     12.  DESCRIPTIVE HEADINGS AND GOVERNING LAW.  The description headings of
the several sections and paragraphs of this Warrant are inserted for convenience
only and do not constitute a part of this Warrant. This Warrant shall be
construed and enforced in accordance with, and the rights of the parties shall
be governed by, the laws of the State of California.

     13.  LOST WARRANTS.  The Company represents and warrants to the Holder
hereof that upon receipt of evidence reasonably satisfactory to the Company of
the loss, theft, destruction, or mutilation of this Warrant and, in the case of
any such loss, theft or destruction, upon receipt of an indemnity reasonably
satisfactory to the Company, or in the case of any such mutilation upon
surrender and cancellation of such Warrant, the Company, at its expense, will
make and deliver a new Warrant, of like tenor, in lieu of the lost, stolen,
destroyed or mutilated Warrant.

     14.  FRACTIONAL SHARES.  No fractional New Securities shall be issued upon
exercise of this Warrant. The Company shall, in lieu of issuing any fractional
New Security, pay the holder entitled to such fraction a sum in cash equal to
such fraction multiplied by the then effective New Security Price.

                                       6.
<PAGE>

     IN WITNESS WHEREOF, the Company has caused this Warrant to be duly executed
by its officers, thereunto duly authorized this 2nd day of April, 1999.

                                    HEALTHCARE TRANSACTION SYSTEMS, INC.
                                    a Delaware corporation


                                    By:_____________________________________

                                    Title:__________________________________



ATTEST:


_________________________________
Secretary

                                       7.
<PAGE>

                                   EXHIBIT A

                               SUBSCRIPTION FORM

                                                              Date:  ___________

Healthcare Transaction Systems, Inc.

______________________________
______________________________
Attn:  President

Ladies and Gentlemen:

[_]    The undersigned hereby elects to exercise the warrant issued to it by
       Healthcare Transaction Systems, Inc. (the "Company") and dated April 2,
       1999, Warrant No. W-001 (the "Warrant") and to purchase thereunder
       __________________________________ New Securities of the Company at a
       purchase price of ______________________________ Dollars ($__________)
       per Security or an aggregate purchase price of
       ______________________________ Dollars ($__________) (the "Purchase
       Price").

       Pursuant to the terms of the Warrant the undersigned has delivered the
Purchase Price herewith in full in cash or by certified check or wire transfer.

                                            Very truly yours,

                                            _________________________________

                                            By:______________________________

                                            Title:___________________________

                                       8.