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                                                                   EXHIBIT 10.20

                         STOCK PURCHASE PROMISSORY NOTE

                                                                October 15, 2001

                  FOR VALUE RECEIVED, _____________ ("MAKER"), having a mailing
address of ___________________ promises to pay to the order of Advancis
Pharmaceutical Corporation, a Delaware corporation ("PAYEE"), having a mailing
address of 942 Clopper Road, Gaithersburg, Maryland 20878, Attention: Vice
President, the principal sum of _________________ Dollars ( ) (the "ORIGINAL
PRINCIPAL BALANCE"), together with interest until paid, as set forth in this
Note.

                  1.       Interest Rate. Interest shall accrue and be payable
on the outstanding unpaid principal balance of this Note at the fixed interest
rate of five and one-half percent (5.50%) per annum.

                  2.       Annual Interest Payments. Maker shall make payments
of accrued and unpaid interest on this Note annually in arrears commencing on
October 15, 2002 and on the same day of each year thereafter during the term of
this Note (each such date is referred to in this Note as an "ANNUAL INTEREST
PAYMENT DATE"). The amount of the interest payment (based on the Original
Principal Balance) due on each Annual Interest Payment Date will be $___________
(the "ANNUAL INTEREST PAYMENT AMOUNT"). The amount of the interest payments will
decrease if principal payments are made on this Note.

                  3.       Principal Payments; Prepayments. Unless sooner paid
in full, and subject to Payee's rights hereunder to require accelerated payment
of this Note upon the occurrence of an Event of Default, the entire unpaid
principal balance of this Note, together with all outstanding and unpaid accrued
interest thereon, shall be due and payable on October 15, 2006 (the "SCHEDULED
MATURITY DATE"). Maker may prepay this Note in whole or in part at any time
without penalty.

                  4.       Manner of Payment; Application of Payments. All
payments shall be made in U.S. dollars in immediately available funds without
set-off or counterclaim or deduction of any kind on the due dates of such
payments, and shall be made to the address for notices to Payee. Payments by
check shall be accepted subject to collection in immediately available funds.
Unless otherwise agreed, payments shall be applied as follows: first to
Collection Costs (defined below in this Note) not previously collected from
Maker by Payee; second to accrued and unpaid interest on this Note; and third to
the principal balance of this Note.

                  5.       Default; Acceleration. The occurrence of any of the
following events shall be an "EVENT OF DEFAULT" under this Note and the Pledge
Agreement:

                           (a)      If Maker shall fail to make a payment of
                                    principal or interest under this Note when
                                    due, and such failure shall continue more
                                    than ten (10) days after Payee gives Maker
                                    written notice thereof; or

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                           (b)      If Maker shall fail to make any other
                                    payment, or fail to perform any other
                                    obligation of Maker, under this Note or the
                                    Pledge Agreement, other than obligations
                                    within the scope of clause (a) of this
                                    Section, and such failure shall continue
                                    more than ten (10) days after Payee gives
                                    Maker written notice thereof; or

                           (c)      If there shall be filed by or against Maker
                                    any petition under the United States
                                    Bankruptcy Code or any similar federal or
                                    state statute; or

                           (d)      Commencement of any proceeding under any
                                    federal or state statute or rule providing
                                    for the relief of debtors, composition of
                                    creditors, arrangement, reorganization,
                                    receivership, liquidation or any similar
                                    event by or against Maker.

Upon the occurrence of an Event of Default, the unpaid principal with interest
and all other sums evidenced by this Note shall, at the option of Payee and in
Payee's discretion, become immediately due and payable. Upon the occurrence of
an Event of Default the Payee shall also have Payee's rights and remedies
available under the Pledge Agreement and any other rights or remedies available
at law or equity.

                  6.       Collateral. This Note is secured by a Pledge
Agreement which covers the property purchased with the credit evidenced by this
Note (the "PLEDGE AGREEMENT").

                  7.       Collection Costs. If Maker shall default in payment
of this Note, or under the Pledge Agreement which secures this Note, and Payee
refers this Note to an attorney who is not a salaried employee of Payee for
collection, Payee may charge and collect from Maker reasonable attorneys fees
and all court costs and other collection costs actually incurred by Payee
relating to Maker's default (such attorneys fees and costs are referred to in
this Note as "COLLECTION COSTS").

                  8.       Notices. Any notice required or permitted by or in
connection with this Note shall be in writing and shall be made by hand
delivery, or by overnight delivery service, or by certified mail, return receipt
requested, postage prepaid, addressed to the parties at the appropriate address
set forth on the first page of this Note or to such other address as may be
hereafter specified by written notice by the parties to each other. Notice shall
be considered given as of the earlier of the date of actual receipt, or the date
of the hand delivery, or one (1) business day after delivery to an overnight
delivery service, or three (3) business days after the date of mailing,
independent of the date of actual delivery or whether delivery is ever in fact
made, as the case may be, provided the giver of notice can establish that notice
was given as provided herein. Notwithstanding the aforesaid procedures, any
notice or demand upon Maker, in fact received by Maker, shall be sufficient
notice or demand.

                  9.       Certain Waivers. As to this Note, Maker waives all
applicable exemption rights, whether under any state constitution, homestead
laws or otherwise, and also waives valuation and appraisement, presentment,
notice of dishonor, and protest, notice of demand and

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nonpayment of this Note, and notice of acceleration and expressly agrees that
the maturity of this Note, or any payment under this Note, may be extended from
time to time without in any way affecting the liability of Maker.

                  10.      Preservation of Payee Rights; Amendments;
Severability. No failure on the part of Payee to exercise any right or remedy
hereunder, whether before or after the happening of an Event of Default shall
constitute a waiver thereof, and no waiver of any past Event of Default shall
constitute waiver of any future default or of any other Event of Default. No
failure to accelerate the indebtedness evidenced hereby by reason of any Event
of Default hereunder, or acceptance of a past due payment, or indulgence granted
from time to time, shall be construed to be a waiver of the right to insist upon
prompt payment thereafter, or shall be deemed to be a novation of this Note or
as a reinstatement of the indebtedness evidenced hereby or as a waiver of such
right or acceleration or any other right, or be construed so as to preclude the
exercise of any right that Payee may have, whether by the laws of the State of
Maryland, by agreement, or otherwise; Maker hereby expressly waives the benefit
of any statute or rule of law or equity that would produce a result contrary to
or in conflict with the foregoing. This Note may not be amended orally, but only
by an agreement in writing signed by the party against whom such agreement is
sought to be enforced. In case any provision or any part of any provision
contained in this Note shall for any reason be held to be invalid, illegal, or
unenforceable in any respect, such invalidity, illegality, or unenforceability
shall not affect any other provision or remaining part of the affected provision
of this Note, but this Note shall be construed as if such invalid, illegal, or
unenforceable provision or part thereof had never been contained herein but only
to the extent such provision or part thereof is invalid, illegal, or
unenforceable.

                  11.      Maryland Law; Jurisdiction; Venue. This Note shall be
governed by the laws of the State of Maryland (excluding Maryland conflicts of
laws rules). Maker hereby irrevocably consents to the non-exclusive personal
jurisdiction of the courts of the State of Maryland and, if a basis for federal
jurisdiction exists, the non-exclusive jurisdiction of the United States
District Court for the District of Maryland. Maker agrees that venue shall be
proper in any circuit court of the State of Maryland selected by Payee or, if a
basis for federal jurisdiction exists, in any Division of the United States
District Court for the District of Maryland. Maker waives any right to object to
the maintenance of any suit or claim in any of the state or federal courts of
the State of Maryland on the basis of improper venue or of inconvenience of
forum. Any suit or claim brought by Maker against Payee that is based, in whole
or in part, directly or indirectly, on this Note or any matters relating to this
Note, shall be brought in a court only in the State of Maryland. Maker shall not
file any counterclaim against Payee in any suit or claim brought by Payee
against Maker in a jurisdiction outside of the State of Maryland unless under
the rules of the court in which Payee brought such suit or claim the
counterclaim is mandatory, and not permissive, and would be considered waived
unless filed as a counterclaim in the claim or suit instituted by Payee against
Maker. Maker agrees that any forum outside the State of Maryland is an
inconvenient forum and that a suit brought by Maker against Payee in any court
outside the State of Maryland should be dismissed or transferred to a court
located in the State of Maryland.

                  12.      Credit Purpose. This Note evidences Maker's
obligation to pay to Payee the purchase price of stock of Payee (the "OPTION
SHARES") purchased by Maker in accordance with

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the terms of a Stock Option Agreement (Form 2) (including all exhibits thereto)
between Maker and Payee (the "STOCK OPTION AGREEMENT"). Maker had the option to
pay cash for the Option Shares, but chose to pay in accordance with the terms of
this Note.

                  13.      Payee's Right of Set-off Upon Payee's Repurchase of
Option Shares. In the event that Payee, in Payee's discretion, shall exercise
Payee's right to repurchase any or all of the Option Shares from Maker in
accordance with the terms of the Stock Option Agreement, Maker agrees that Payee
shall have the right (but not the obligation) to offset the amount of the
purchase price owed by Payee to Maker for the purchase price of the repurchased
Option Shares against amounts then owed by Maker to Payee under this Note. Any
such offset described in the preceding sentence shall be deemed to be a cash
payment by Payee to Maker under the terms of the Stock Option Agreement in the
amount of the purchase price of the repurchased Option Shares offset.

                  IN WITNESS WHEREOF, and intending to be legally bound hereby
Maker executes this Note under seal.

WITNESS:

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                          (SEAL)
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                                          Name:

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