EX - 10.23


July 10, 1998



John S. Senaldi
1243 Laurel Street, Apt. C
Menlo Park, California  94025

Dear John:

To confirm  our  conversation,  I am pleased to offer you the  position  of Vice
President  Worldwide  Marketing for Molecular Devices. In this position you will
report directly to me.

Your  base  salary  will be  $13,335  per month  ($160,020  per  annum)  payable
semi-monthly on the 15th and last day of each month. Upon your start date you be
provided a one-time bonus of $30,000.

As a member of the management  staff, you will be eligible to participate in the
1998 MDC Executive Bonus Plan (bonus at Plan 40% prorated for time in position).
The Company will guarantee you a minimum bonus of $30,000 under the plan for the
1998 plan year,  which will be paid in the first quarter of 1999.  You will also
receive all the employment  benefits available to regular full-time employees of
Molecular Devices.

You will be eligible to receive 46,000  Incentive  Stock Options  subject to the
approval of the Board of Directors.  These options will vest in accordance  with
the Company's  vesting schedule over a period of 5 years, 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  2,500 shares of  Restricted  Stock,  subject to  applicable
securities law restrictions.  These options will vest quarterly over a period of
two years.

In the event of a Change of  Control,  all your stock  options  and shares  will
become fully vested at such date.

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.

<PAGE>

July 10, 1998
Page two


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.

John,  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

Attachments

Accepted:            John Senaldi//            
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Date:                 July 13, 1998            
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Start Date:                                    
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JAD/jd