AKZO NOBEL


BY TELEFAX: +1 510 845-7914
AND BY COURIER
Ronald S. Barkin
President
BioTime, Inc.
935 Pardee Street
Berkeley, CA 94710


                                    February 13, 2001


Re:      Exclusive License Agreement entered into as of February 13, 2001
         between BioTime, Inc. and Horus B.V.

Dear Mr. Barkin:

We hereby  certify  that  Horus  B.V.  of  Boxtel,  the  Netherlands,  is at the
effective  date  of the  above  referenced  Exclusive  License  Agreement  ("the
Agreement") part of the Akzo Nobel group of companies.

In  consideration of the Agreement  between  BioTime,  Inc. and Horus B.V., Akzo
Nobel N.V., as the ultimate  corporate  parent of Horus B.V., by its  authorized
signature  below  hereby  guarantees  to  BioTime,  Inc.  all of the payment and
performance  obligations  by Horus B.V. (and any of its  successors  and assigns
within the Akzo Nobel group of companies)  under the  Agreement  when and as the
performance or payment of such obligations  becomes due, in the event Horus B.V.
is unable or fails to perform.

This guaranty applies to the Agreement as in effect on the date of this guaranty
and as the Agreement may be amended,  modified, or supplemented in the future by
BioTime,  Inc.,  and Horus  B.V.  Akzo Nobel N.V.  agrees  that any  supplement,
modification, amendment, extension, renewal, acceleration or other change of one
or more terms of the  Agreement  and the  obligations  of Horus B.V. or BioTime,
Inc.  under the  Agreement  shall not affect the  enforceability  or  continuing
effectiveness of this guaranty even if executed or implemented by BioTime,  Inc.
and Horus B.V. without notice to or the consent or approval of Akzo Nobel N.V.

This guaranty may not be revoked,  amended,  terminated, or assigned without the
written  consent of BioTime,  Inc., so long as Horus B.V. (or its  successors or
assignee companies) is a company within the Akzo Nobel group of companies.


<PAGE>

In the  event  that at any  time  Horus  B.V.  (or its  successors  or  assignee
companies)  ceases  being a member of the Akzo Nobel  group of  companies,  this
guaranty will lapse in respect of obligations  accrued under the Agreement after
the date of such event, provided that Akzo Nobel N.V. at the request of BioTime,
Inc., will spend all reasonable  efforts to have a similar guaranty given by the
new ultimate parent company (if any) of Horus B.V. (or its successor or assignee
company).

BioTime,  Inc. may deliver any notices under this guaranty to Akzo Nobel N.V. at
the following address by personal  delivery,  by facsimile  confirmed by postage
prepaid  first-class mail, by over-night or next day business air courier, or by
postage prepaid certified mail to the following address:

         Akzo Nobel N.V.
         PO Box 9300
         6800 SB Arnhem, the Netherlands
         Attention:  General Counsel

Notices shall be effective upon receipt if personally  delivered or delivered by
facsimile or air courier,  or on the third  business day  following  the date of
mailing.

Akzo  Nobel  N.V.  may  change its  address  listed  above by written  notice to
BioTime,  Inc.,  at the  following  address or at such other address as BioTime,
Inc. may provide to Akzo Nobel N.V. in writing:

         Bio Time, Inc.
         935 Pardee Street
         Berkeley, CA 94710
         Attention:  President


AKZO NOBEL N.V.


   By: /s/F.  Frohlich                           By:   /s/L.  Lohrens
      -----------------------------                  --------------------------

 Name: F.  Frohlich                            Name:   L.  Lohrens
      -----------------------------                  --------------------------

Title: CF0                                    Title:   Director Control Pharma
      -----------------------------                  --------------------------