Document:

<PAGE>

                                  EXHIBIT 10.21

<PAGE>

                                   EXHIBIT C-6

                                 PROMISSORY NOTE

$74,250                                                        San Francisco, CA

                                                                    FEB 21, 2000

         FOR VALUE RECEIVED, MICHAEL S. OSTRACH promises to pay to Kosan
Biosciences Incorporated (the "Company"), or order, the principal sum of
SEVENTY-FOUR THOUSAND TWO HUNDRED FIFTY ($74,250 ), together with interest on
the unpaid principal hereof from the date hereof at the rate of SIX AND 46/100
percent (6.46%) per annum, compounded semiannually.

         Principal and interest shall be due and payable on FEB 21, 2003.
Should the undersigned fail to make full payment of principal or interest for a
period of 10 days or more after the due date thereof, the whole unpaid balance
on this Note of principal and interest shall become immediately due at the
option of the holder of this Note. Payments of principal and interest shall be
made in lawful money of the United States of America.

         The undersigned may at any time prepay all or any portion of the
principal or interest owing hereunder.

         This Note is subject to the terms of the Option, dated as of JAN. 20,
1998 & OCT 23, 1998. This Note is secured in part by a pledge of the Company's
Common Stock under the terms of a Security Agreement of even date herewith and
is subject to all the provisions thereof.

         The holder of this Note shall have full recourse against the
undersigned, and shall not be required to proceed against the collateral
securing this Note in the event of default.

         In the event the undersigned shall cease to be an employee or
consultant of the Company for any reason, this Note shall, at the option of the
Company, be accelerated, and the whole unpaid balance on this Note of principal
and accrued interest shall be immediately due and payable.

         Should any action be instituted for the collection of this Note, the
reasonable costs and attorneys' fees therein of the holder shall be paid by the
undersigned.

                                             /s/ Michael Ostrach
                                             ----------------------------------
                                             Signature

                                             Michael Ostrach
                                             ----------------------------------
                                             Print Name

<PAGE>

                                   EXHIBIT C-7

                               SECURITY AGREEMENT

         This Security Agreement is made as of FEB 21, 2000 between Kosan
Biosciences Incorporated ("Pledgee"), and MICHAEL S. OSTRACH ("Pledgor").

                                    RECITALS

         Pursuant to Pledgor's election to purchase Shares under the Option
Agreement dated JAN. 20, 1998 & OCT. 23, 1998, (the "Option"), between Pledgor
and Pledgee under Pledgee's 1996 Stock Option Plan, and Pledgor's election under
the terms of the Option to pay for such shares with his promissory note (the
"Note"), Pledgor has purchased 110,000 shares of Pledgee's Common Stock (the
"Shares") at a price of $0.45 & 1.00 per share, for a total purchase price of
$74,250 . The Note and the obligations thereunder are as set forth in Exhibit
C-6 to the Option.

         NOW, THEREFORE, it is agreed as follows:

         1.       CREATION AND DESCRIPTION OF SECURITY INTEREST. In
consideration of the transfer of the Shares to Pledgor under the Option
Agreement, Pledgor, pursuant to the California Commercial Code, hereby pledges
all of such Shares (herein sometimes referred to as the "Collateral")
represented by certificate number____, duly endorsed in blank or with executed
stock powers, and herewith delivers said certificate to the Secretary of Pledgee
("Pledgeholder"), who shall hold said certificate subject to the terms and
conditions of this Security Agreement.

         The pledged stock (together with an executed blank stock assignment for
use in transferring all or a portion of the Shares to Pledgee if, as and when
required pursuant to this Security Agreement) shall be held by the Pledgeholder
as security for the repayment of the Note, and any extensions or renewals
thereof, to be executed by Pledgor pursuant to the terms of the Option, and the
Pledgeholder shall not encumber or dispose of such Shares except in accordance
with the provisions of this Security Agreement.

         2.       PLEDGOR'S REPRESENTATIONS AND COVENANTS. To induce Pledgee to
enter into this Security Agreement, Pledgor represents and covenants to Pledgee,
its successors and assigns, as follows:

                  a.       PAYMENT OF INDEBTEDNESS. Pledgor will pay the
principal sum of the Note secured hereby, together with interest thereon, at the
time and in the manner provided in the Note.

                  b.       ENCUMBRANCES. The Shares are free of all other
encumbrances, defenses and liens, and Pledgor will not further encumber the
Shares without the prior written consent of Pledgee.

                  c.       MARGIN REGULATIONS. In the event that Pledgee's
Common Stock is now or later becomes margin-listed by the Federal Reserve Board
and Pledgee is classified as a "lender" within the meaning of the regulations
under Part 207 of Title 12 of the Code of Federal Regulations ("Regulation G"),
Pledgor agrees to cooperate with Pledgee in making any amendments to the Note or
providing any additional collateral as may be necessary to comply with such
regulations.

<PAGE>

         3.       VOTING RIGHTS. During the term of this pledge and so long as
all payments of principal and interest are made as they become due under the
terms of the Note, Pledgor shall have the right to vote all of the Shares
pledged hereunder.

         4.       STOCK ADJUSTMENTS. In the event that during the term of the
pledge any stock dividend, reclassification, readjustment or other changes are
declared or made in the capital structure of Pledgee, all new, substituted and
additional shares or other securities issued by reason of any such change shall
be delivered to and held by the Pledgee under the terms of this Security
Agreement in the same manner as the Shares originally pledged hereunder. In the
event of substitution of such securities, Pledgor, Pledgee and Pledgeholder
shall cooperate and execute such documents as are reasonable so as to provide
for the substitution of such Collateral and, upon such substitution, references
to "Shares" in this Security Agreement shall include the substituted shares of
capital stock of Pledgor as a result thereof.

         5.       OPTIONS AND RIGHTS. In the event that, during the term of this
pledge, subscription Options or other rights or options shall be issued in
connection with the pledged Shares, such rights, Options and options shall be
the property of Pledgor and, if exercised by Pledgor, all new stock or other
securities so acquired by Pledgor as it relates to the pledged Shares then held
by Pledgeholder shall be immediately delivered to Pledgeholder, to be held under
the terms of this Security Agreement in the same manner as the Shares pledged.

         6.       DEFAULT. Pledgor shall be deemed to be in default of the Note
and of this Security Agreement in the event:

                  a.       Payment of principal or interest on the Note shall be
delinquent for a period of 10 days or more; or

                  b.       Pledgor fails to perform any of the covenants set
forth in the Option or contained in this Security Agreement for a period of 10
days after written notice thereof from Pledgee.

         In the case of an event of Default, as set forth above, Pledgee shall
have the right to accelerate payment of the Note upon notice to Pledgor, and
Pledgee shall thereafter be entitled to pursue its remedies under the California
Commercial Code.

         7.       RELEASE OF COLLATERAL. Subject to any applicable contrary
rules under Regulation G, there shall be released from this pledge a portion of
the pledged Shares held by Pledgeholder hereunder upon payments of the principal
of the Note. The number of the pledged Shares which shall be released shall be
that number of full Shares which bears the same proportion to the initial number
of Shares pledged hereunder as the payment of principal bears to the initial
full principal amount of the Note.

         8.       WITHDRAWAL OR SUBSTITUTION OF COLLATERAL. Pledgor shall not
sell, withdraw, pledge, substitute or otherwise dispose of all or any part of
the Collateral without the prior written consent of Pledgee.

         9.       TERM. The within pledge of Shares shall continue until the
payment of all indebtedness secured hereby, at which time the remaining pledged
stock shall be promptly delivered to Pledgor,

                                      -2-
<PAGE>

subject to the provisions for prior release of a portion of the Collateral as
provided in paragraph 7 above.

         10.      INSOLVENCY. Pledgor agrees that if a bankruptcy or insolvency
proceeding is instituted by or against it, or if a receiver is appointed for the
property of Pledgor, or if Pledgor makes an assignment for the benefit of
creditors, the entire amount unpaid on the Note shall become immediately due and
payable, and Pledgee may proceed as provided in the case of default.

         11.      PLEDGEHOLDER LIABILITY. In the absence of willful or gross
negligence, Pledgeholder shall not be liable to any party for any of his acts,
or omissions to act, as Pledgeholder.

         12.      INVALIDITY OF PARTICULAR PROVISIONS. Pledgor and Pledgee agree
that the enforceability or invalidity of any provision or provisions of this
Security Agreement shall not render any other provision or provisions herein
contained unenforceable or invalid.

         13.      SUCCESSORS OR ASSIGNS. Pledgor and Pledgee agree that all of
the terms of this Security Agreement shall be binding on their respective
successors and assigns, and that the term "Pledgor" and the term "Pledgee" as
used herein shall be deemed to include, for all purposes, the respective
designees, successors, assigns, heirs, executors and administrators.

         14.      GOVERNING LAW. This Security Agreement shall be interpreted
and governed under the laws of the State of California.

         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year first above written.

         "PLEDGOR"                 By:     /s/ Michael Ostrach
                                      ------------------------------------------

                                   M OSTRACH
                                   ---------------------------------------------
                                   Print Name

                                   Address: 15 Honeywood Rd
                                           -------------------------------------
                                            Orinda, CA 94567
                                           -------------------------------------

         "PLEDGEE"                 Kosan Biosciences Incorporated

                                   By:      /s/ Kevin Kaster
                                      ------------------------------------------
                                   Title:   VICE PRESIDENT
                                         ---------------------------------------

         "PLEDGEHOLDER"            /s/ Kevin Kaster
                                   ---------------------------------------------
                                   Stock Option Administrator of
                                   Kosan Biosciences Incorporated

                                      -3-<PAGE>

                                  EXHIBIT 10.22

<PAGE>

                                   EXHIBIT C-6

                                 PROMISSORY NOTE

$ 50,000.00                                                    San Francisco, CA

                                                               FEBRUARY 21, 2000

         FOR VALUE RECEIVED, SUSAN KANAYA promises to pay to Kosan Biosciences
Incorporated (the "Company"), or order, the principal sum of FIFTY THOUSAND
DOLLARS ($50,000.00/XX), together with interest on the unpaid principal hereof
from the date hereof at the rate of SIX AND FORTY SIX ONE HUNDREDTHS percent
(6.46%) per annum, compounded semiannually.

         Principal and interest shall be due and payable on FEBRUARY 21, 2003.
Should the undersigned fail to make full payment of principal or interest for a
period of 10 days or more after the due date thereof, the whole unpaid balance
on this Note of principal and interest shall become immediately due at the
option of the holder of this Note. Payments of principal and interest shall be
made in lawful money of the United States of America.

         The undersigned may at any time prepay all or any portion of the
principal or interest owing hereunder.

         This Note is subject to the terms of the Option, dated as of NOVEMBER
4, 1999. This Note is secured in part by a pledge of the Company's Common Stock
under the terms of a Security Agreement of even date herewith and is subject to
all the provisions thereof.

         The holder of this Note shall have full recourse against the
undersigned, and shall not be required to proceed against the collateral
securing this Note in the event of default.

         In the event the undersigned shall cease to be an employee or
consultant of the Company for any reason, this Note shall, at the option of the
Company, be accelerated, and the whole unpaid balance on this Note of principal
and accrued interest shall be immediately due and payable.

         Should any action be instituted for the collection of this Note, the
reasonable costs and attorneys' fees therein of the holder shall be paid by the
undersigned.

                                    /s/ Susan Kanaya
                                    -------------------------------------------
                                    Signature

                                    SUSAN M. KANAYA
                                    -------------------------------------------
                                    Print Name

<PAGE>

                                   EXHIBIT C-7

                               SECURITY AGREEMENT

         This Security Agreement is made as of FEBRUARY 21, 2000 between Kosan
Biosciences Incorporated ("Pledgee"), and SUSAN M. KANAYA ("Pledgor").

                                    RECITALS

         Pursuant to Pledgor's election to purchase Shares under the Option
Agreement dated NOV. 04, 1999 (the "Option"), between Pledgor and Pledgee under
Pledgee's 1996 Stock Option Plan, and Pledgor's election under the terms of the
Option to pay for such shares with his promissory note (the "Note"), Pledgor has
purchased 50,000 shares of Pledgee's Common Stock (the "Shares") at a price of
$1.00 per share, for a total purchase price of $50,000.00. The Note and the
obligations thereunder are as set forth in Exhibit C-6 to the Option.

         NOW, THEREFORE, it is agreed as follows:

         1.       CREATION AND DESCRIPTION OF SECURITY INTEREST. In
consideration of the transfer of the Shares to Pledgor under the Option
Agreement, Pledgor, pursuant to the California Commercial Code, hereby
pledges all of such Shares (herein sometimes referred to as the "Collateral")
represented by certificate number_______, duly endorsed in blank or with
executed stock powers, and herewith delivers said certificate to the
Secretary of Pledgee ("Pledgeholder"), who shall hold said certificate
subject to the terms and conditions of this Security Agreement.

         The pledged stock (together with an executed blank stock assignment for
use in transferring all or a portion of the Shares to Pledgee if, as and when
required pursuant to this Security Agreement) shall be held by the Pledgeholder
as security for the repayment of the Note, and any extensions or renewals
thereof, to be executed by Pledgor pursuant to the terms of the Option, and the
Pledgeholder shall not encumber or dispose of such Shares except in accordance
with the provisions of this Security Agreement.

         2.       PLEDGOR'S REPRESENTATIONS AND COVENANTS. To induce Pledgee to
enter into this Security Agreement, Pledgor represents and covenants to Pledgee,
its successors and assigns, as follows:

                  a.       PAYMENT OF INDEBTEDNESS. Pledgor will pay the
principal sum of the Note secured hereby, together with interest thereon, at the
time and in the manner provided in the Note.

                  b.       ENCUMBRANCES. The Shares are free of all other
encumbrances, defenses and liens, and Pledgor will not further encumber the
Shares without the prior written consent of Pledgee.

                  c.       MARGIN REGULATIONS. In the event that Pledgee's
Common Stock is now or later becomes margin-listed by the Federal Reserve Board
and Pledgee is classified as a "lender" within the meaning of the regulations
under Part 207 of Title 12 of the Code of Federal Regulations ("Regulation G"),
Pledgor agrees to cooperate with Pledgee in making any amendments to the Note or
providing any additional collateral as may be necessary to comply with such
regulations.

<PAGE>

         3.       VOTING RIGHTS. During the term of this pledge and so long as
all payments of principal and interest are made as they become due under the
terms of the Note, Pledgor shall have the right to vote all of the Shares
pledged hereunder.

         4.       STOCK ADJUSTMENTS. In the event that during the term of the
pledge any stock dividend, reclassification, readjustment or other changes are
declared or made in the capital structure of Pledgee, all new, substituted and
additional shares or other securities issued by reason of any such change shall
be delivered to and held by the Pledgee under the terms of this Security
Agreement in the same manner as the Shares originally pledged hereunder. In the
event of substitution of such securities, Pledgor, Pledgee and Pledgeholder
shall cooperate and execute such documents as are reasonable so as to provide
for the substitution of such Collateral and, upon such substitution, references
to "Shares" in this Security Agreement shall include the substituted shares of
capital stock of Pledgor as a result thereof.

         5.       OPTIONS AND RIGHTS. In the event that, during the term of this
pledge, subscription Options or other rights or options shall be issued in
connection with the pledged Shares, such rights, Options and options shall be
the property of Pledgor and, if exercised by Pledgor, all new stock or other
securities so acquired by Pledgor as it relates to the pledged Shares then held
by Pledgeholder shall be immediately delivered to Pledgeholder, to be held under
the terms of this Security Agreement in the same manner as the Shares pledged.

         6.       DEFAULT. Pledgor shall be deemed to be in default of the Note
and of this Security Agreement in the event:

                  a.       Payment of principal or interest on the Note
shall be delinquent for a period of 10 days or more; or

                  b.       Pledgor fails to perform any of the covenants set
forth in the Option or contained in this Security Agreement for a period of 10
days after written notice thereof from Pledgee.

         In the case of an event of Default, as set forth above, Pledgee shall
have the right to accelerate payment of the Note upon notice to Pledgor, and
Pledgee shall thereafter be entitled to pursue its remedies under the California
Commercial Code.

         7.       RELEASE OF COLLATERAL. Subject to any applicable contrary
rules under Regulation G, there shall be released from this pledge a portion of
the pledged Shares held by Pledgeholder hereunder upon payments of the principal
of the Note. The number of the pledged Shares which shall be released shall be
that number of full Shares which bears the same proportion to the initial number
of Shares pledged hereunder as the payment of principal bears to the initial
full principal amount of the Note.

         8.       WITHDRAWAL OR SUBSTITUTION OF COLLATERAL. Pledgor shall not
sell, withdraw, pledge, substitute or otherwise dispose of all or any part of
the Collateral without the prior written consent of Pledgee.

         9.       TERM. The within pledge of Shares shall continue until the
payment of all indebtedness secured hereby, at which time the remaining pledged
stock shall be promptly delivered to Pledgor,

                                      -2-
<PAGE>

subject to the provisions for prior release of a portion of the Collateral as
provided in paragraph 7 above.

         10.      INSOLVENCY. Pledgor agrees that if a bankruptcy or insolvency
proceeding is instituted by or against it, or if a receiver is appointed for the
property of Pledgor, or if Pledgor makes an assignment for the benefit of
creditors, the entire amount unpaid on the Note shall become immediately due and
payable, and Pledgee may proceed as provided in the case of default.

         11.      PLEDGEHOLDER LIABILITY. In the absence of willful or gross
negligence, Pledgeholder shall not be liable to any party for any of his acts,
or omissions to act, as Pledgeholder.

         12.      INVALIDITY OF PARTICULAR PROVISIONS. Pledgor and Pledgee agree
that the enforceability or invalidity of any provision or provisions of this
Security Agreement shall not render any other provision or provisions herein
contained unenforceable or invalid.

         13.      SUCCESSORS OR ASSIGNS. Pledgor and Pledgee agree that all of
the terms of this Security Agreement shall be binding on their respective
successors and assigns, and that the term "Pledgor" and the term "Pledgee" as
used herein shall be deemed to include, for all purposes, the respective
designees, successors, assigns, heirs, executors and administrators.

         14.      GOVERNING LAW. This Security Agreement shall be interpreted
and governed under the laws of the State of California.

         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year first above written.

         "PLEDGOR"                    By:      /s/ Susan Kanaya
                                         ---------------------------------------

                                               SUSAN M. KANAYA
                                      ------------------------------------------
                                      Print Name

                                      Address: 1421 WOODBERRY AVE.

                                               SAN MATEO, CA 94403

         "PLEDGEE"                    Kosan Biosciences Incorporated

                                      By:      /s/ Kevin Kaster
                                         ---------------------------------------
                                      Title:   VICE PRESIDENT
                                            ------------------------------------

         "PLEDGEHOLDER"               /s/ Kevin Kaster
                                      ------------------------------------------
                                      Stock Option Administrator of
                                      Kosan Biosciences Incorporated

                                      -3-

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