EX - 10.21


July 7, 1998



Timothy Harkness
36 Rico Way
San Francisco, California  94123

Dear Tim:

To confirm  our  conversation,  I am pleased to offer you the  position  of Vice
President  Finance and Chief Financial  Officer.  We have agreed that your start
date will be July 9, 1998. In this position you will report directly to me.

Upon your start date with Molecular Devices your base salary will be $12,500 per
month ($150,000 per annum) payable semi-monthly on the 15th and last day of each
month.  Your salary will  escalate to $162,600 per annum in three (3) months and
to $175,000 in six (6) months  based upon agreed to milestone  achievements.  In
addition,  you will be  provided a one-time  hiring  bonus of $87,500  upon your
start date with MDC.

As a member of the management  staff, you will be eligible to participate in the
1998 MDC  Executive  Bonus Plan (bonus at Plan 40%,  which will be prorated  for
employment tenure).

You will be eligible to receive 75,000  Incentive  Stock Options  subject to the
approval of the Board of  Directors.  These options will vest over a period of 5
years, with 15,000 vesting on the first anniversary of employment date and 3,750
shares vesting every quarter  thereafter,  and will be subject to the provisions
of the Company's 1995 Stock Option Plan as amended.

Additionally,  subject to the  approval of the Board of  Directors,  you will be
eligible to receive  10,000  shares of the Company's  Common  Stock,  subject to
applicable  securities law restrictions.  A total of 1250 shares will be granted
following each quarter of service. Upon granting,  each individual grant will be
fully earned and vested. You will bear all tax responsibilities for these grants
and will make  arrangements  with the  Company  to ensure  payment  of taxes and
compliance with tax withholdings.  In lieu of your having to sell stock in order
to make any of the required tax payments, the Company agrees to lend you, for up
to  two  (2)  years,  at the  minimum  interest  rate  required  by  the  taxing
authorities to avoid the imputation of taxes,  an amount equal to your state and

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July 7, 1998 
Page two


federal tax withholding  obligation arising from the stock grant, any such loans
to be secured by your stock.

In the event of a Change of Control resulting in either  termination or demotion
("demotion"  means a  reduction  in base  salary,  duties,  title  or  reporting
relationship),  all your stock  options and shares will become  fully  vested at
such  date.  In  addition,  at the  time of your  leaving  the  Company,  at the
Change-of-control  closing  date,  you  would be  granted a  one-time  severance
payment  equal to the last  twelve  months of your total  compensation  (last 12
months salary plus most recent annual bonus awarded to you).

In the event of  termination  "without  cause"  within the first two  years:  a)
granting of 10,000 shares  referred to in paragraph 5 will  accelerate such that
you will  receive  10,000  shares and b) you will  receive a one-time  severance
payment equal to the prior six (6) months compensation.

Termination  "for cause" shall be determined by MDC based on the belief that one
or more of the following  has occurred:  i) indictment or conviction of a felony
ii) fraud against MDC or, iii) intentional damage to MDC property.

Upon your start date, as a condition of employment, you will be required to sign
an  Employee   Confidentiality  and  Inventions  Agreement  (copy  of  agreement
attached).  Please review this  agreement  carefully;  if you have any questions
regarding this agreement, please feel free to call me.

This  offer  is  made   contingent   upon  your  ability  to  provide  proof  of
identification  and authorization to work in the United States.  Upon employment
at  Molecular  Devices,  you will be required to furnish such  documentation  as
described in the enclosed materials.

Additionally,  this written offer  constitutes  all conditions and agreements of
Molecular Devices Corporation. Upon acceptance of this offer, please sign, date,
indicate your start date, and return this letter and the Application Form to the
Human  Resources  Department.  However,  if you have not responded to this offer
within five (5) days of receipt the offer will be withdrawn.

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July 7, 1998
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Tim,  Molecular  Devices  Corporation  is facing many exciting  challenges as we
grow.  Your experience and talents will be strong  additions to our company.  We
are looking forward to having you join our team.

         Sincerely,

         MOLECULAR DEVICES CORPORATION


         Joseph D. Keegan//

         Joseph D. Keegan
         President and Chief Executive Officer

Attachment

Accepted:       Tim Harkness//                 
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Date:            July 8, 1998                  
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Start Date:
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