This debenture has not been registered under the Securities Act of 1933 or
     any applicable state securities laws, and may not be offered for sale,
         sold, transferred, pledged or hypothecated without an effective
    registration statement under the Securities Act and under any applicable
        state securities laws, or an opinion of counsel, satisfactory to
        Borrower, that an exemption from such registration is available.

                                    DEBENTURE

Series 2001-A                                            Dated: August __, 2001
Certificate No. __
Amount $___________


         FOR  GOOD  AND  VALUABLE  CONSIDERATION,  BioTime,  Inc.  a  California
corporation  ("Borrower"),  promises to pay to the order of _________________ or
the  subsequent  registered  holder the principal sum of  ___________________AND
NO/100 DOLLARS  ($__________),  together with interest thereon,  all as provided
below:

         1. Series 2001-A Debenture.  This Debenture is one of a duly authorized
series of  Debentures  of like tenor and effect  issued by Borrower  referred to
herein as the "Series 2001-A  Debentures." Up to $5,000,000 of the Series 2001-A
Debentures (of which this Debenture is a part) may be issued by Borrower.

         2. Equal Rank.  All Series 2001-A  Debentures  rank equally and ratably
without priority over one another.

         3. Maturity.  Subject to the other  provisions of this  Debenture,  the
principal amount of this Debenture shall be due and payable in full on August 1,
2004 (the "Maturity Date").

         4. Manner of  Payment/Crediting  of  Payments.  The  Borrower  will pay
interest  and  principal on this  Debenture to the person who is the  registered
holder  of this  Debenture  at the  close of  business  on the date  immediately
preceding the next interest  payment date  specified in this  Debenture,  or the
Maturity Date, as applicable,  even if such Debenture is canceled or transferred
on the interest  payment date or Maturity  Date. The Borrower will pay principal
and interest in money of the United  States that at the time of payment is legal
tender for payment of public and private  debts.  However,  the Borrower may pay
principal and interest by check payable in such money, and any such check may be
mailed to a holder's registered address.

         5. Interest.


              (a) Interest on the principal amount of this Debenture outstanding
from time to time shall  accrue at the rate of ten  percent  (10.00%)  per annum
(computed on the basis of a 365 day year).  Interest  accrued on this  Debenture
shall be due and payable on February 1 and August 1 each year.


                                        1






              (b) In the event that any payment of  principal or interest is not
paid within five (5) days from on the date on which the same is due and payable,
such payment  shall  continue as an  obligation  of the  Borrower,  and interest
thereon from the due date of such payment shall accrue until paid in full at the
lesser of (i) fifteen percent (15%) per annum, or (ii) the highest interest rate
permitted under applicable law (the "Default Rate"). From and after the Maturity
Date or upon acceleration of this Debenture, the entire unpaid principal balance
with all unpaid interest accrued thereon, and any and all other fees and charges
then due at such maturity, shall bear interest at the Default Rate.

         6. Prepayment.  Principal and interest accrued on this Debenture may be
prepaid  in part or in full at any  time  and  from  time to time at  Borrower's
option, without penalty, premium or discount, upon ten (10) days prior notice to
the holder.

         7. Transfer of Debenture

              (a)  Borrower  may deem and treat the  person or  persons in whose
name this Debenture  shall be registered  upon the books and records of Borrower
as (i) the  absolute  owner  of  this  Debenture  (regardless  of  whether  this
Debenture  shall be past due, and  notwithstanding  any  notation of  ownership,
endorsement, or other writing on this Debenture) and (ii) the person entitled to
receive  payment of or on account of principal and interest due or payable under
this Debenture,  and for all other purposes;  and Borrower shall not be affected
by any notice to the contrary  unless such notice of transfer is given  pursuant
to Section 7(c).  All such payments  shall be valid and effectual to satisfy and
discharge the liability on this Debenture to the extent of all sums so paid.

              (b)  Borrower  shall not be bound to  recognize  any  equitable or
other claim or interest in this  Debenture  on the part of any person other than
the person or persons in whose name this Debenture  shall be registered upon the
books and  records  of  Borrower,  and  Borrower  shall  not be  liable  for any
registration  of  transfer  of  any  Debenture  which  is  registered  or  to be
registered  in the  name of a  fiduciary  or the  nominee  of a  fiduciary  upon
instruction  from such fiduciary,  unless made with the actual  knowledge that a
fiduciary  or  nominee  is  committing  a breach  of trust  in  requesting  such
registration  or  transfer,  or with  such  knowledge  of such  facts  that  its
participation therein amounts to bad faith.

              (c)  The  transfer  of  this   Debenture  is  registrable  by  the
registered holder in person,  or by his attorney duly authorized in writing,  on
the books and records of the Borrower at the address designated in Section 7(d),
as such address may be changed from time to time as provided therein, subject to
the terms, conditions, and restrictions of transfer set forth in this Debenture,
but  without  payment of any charge  other than a sum  sufficient  to  reimburse
Borrower  for  any tax or  other  governmental  charge  incident  thereto.  Such
registration  of  transfer  shall be  effected  only  upon  compliance  with the
provisions  of this  Section  7, and upon  surrender  and  cancellation  of this
Debenture. Upon any such registration of transfer, a new Debenture or Debentures
of the same aggregate  principal  amount will be issued to the persons  entitled
thereto  in  exchange  for  this   Debenture.   All  Debentures   presented  for
registration of transfer, if so required by Borrower, shall


                                        2




be  accompanied  by a written  instrument or  instruments  of transfer,  in form
reasonably  satisfactory to Borrower,  duly executed by the registered holder or
by his duly authorized  attorney,  with all signatures  acknowledged by a notary
public. In all cases of transfer by an attorney, the original power of attorney,
duly approved, or a copy thereof, duly certified,  shall be deposited and remain
with the Borrower. In case of transfer by executors,  administrators,  guardians
or other legal  representatives,  duly authenticated evidence of their authority
shall be  produced,  and may be  required  to be  deposited  and remain with the
Borrower in its discretion.

              (d)  Borrower  shall keep and  maintain a register or registers in
which  Borrower shall  register  Debentures and the transfer of Debentures.  All
payments of  principal,  interest,  and any other amount due or that becomes due
under this Debenture,  shall be paid to the holder by check mailed to the holder
at his or her  address of record.  The holder may change the address for payment
or for notice by delivery of written notice to Borrower.  The address for notice
to Borrower shall be 935 Pardee Street,  Berkeley,  California 94710; Attention:
Chief Financial  Officer.  Borrower may change its address for notice by written
notice to holder.

              (e)  This  Debenture  may  not  be  sold,  pledged,  hypothecated,
negotiated,  assigned,  or otherwise transferred except pursuant to an effective
registration  statement  under  the  Securities  Act of 1933,  as  amended,  and
pursuant to effective  registration  or  qualification  under  applicable  state
securities or "blue sky" laws,  unless an exemption from such  registration  and
qualification is available.  Borrower will make a stop transfer  notation in the
register maintained pursuant to this Section 7 with respect to such restrictions
on transfer.  Borrower may require,  as a condition to registration of transfer,
that the transferor or transferee deliver to Borrower an opinion of counsel,  in
form and substance  reasonably  acceptable to Borrower,  to the effect that such
transfer is exempt from the  registration  and  qualification  provisions of the
Securities  Act of 1933, as amended,  and applicable  state  securities or "blue
sky" laws.

         8. Default.


              (a)  Subject  to the  provisions  of this  Section  8, the  unpaid
principal balance of this Debenture, together with all accrued interest thereon,
shall, at the option of the holder of this Debenture, become immediately due and
payable upon the  occurrence of any of the  following  events (each an "Event of
Default"):  (i) the default of Borrower in the payment of any interest due under
this Debenture if such default  continues for ten (10) calendar  days;  (ii) the
default of Borrower in the payment of any  principal  due under this  Debenture;
(iii)  the  failure  of  Borrower  for a period  of 30 days  after  notice  from
Debenture  holders to observe or  perform  in any  material  respect  any of the
provisions of this Debenture,  other than payment of interest or principal; (iv)
Borrower instituting or consenting to any proceeding under any Debtor Relief Law
(as defined  below) with respect to Borrower or any part of Borrower's  property
or  assets,  or  Borrower  becoming  the  subject  of any order for  relief in a
proceeding  under any Debtor  Relief  Law;  (v) the  institution  of any case or
proceeding  under any Debtor  Relief Law with respect to Borrower or any part of
Borrower's  property  or assets  without the consent of Borrower if such case or
proceeding continues  undismissed or unstayed for sixty (60) calendar days; (vi)
the issuance or levy of any judgment, writ, warrant of

                                        3




attachment  or  execution  or  similar  process  against  property  or assets of
Borrower  in the  amount of $25,000 or more,  if such  process is not  released,
vacated or fully bonded within 60 calendar  days after its issue or levy;  (vii)
Borrower  making  an  assignment  for  the  benefit  of  creditors;  (viii)  the
dissolution or liquidation of Borrower, or (ix) Borrower defaults in the payment
of principal or interest on any  obligation for money owed in excess of $100,000
if such default  continues  for a period of ten calendar  days.  As used in this
Debenture,  the term "Debtor Relief Law" shall mean the  Bankruptcy  Code of the
United  States of America,  as  amended,  or any other  applicable  liquidation,
conservatorship,    bankruptcy,   moratorium,    rearrangement,    receivership,
insolvency, reorganization, or similar debtor relief law affecting the rights of
creditors generally.

              (b) If an Event of Default  specified in clause (iv),  (v), (vii),
or (viii) of  paragraph  (a) of this  Section 8  occurs,  the  unpaid  principal
balance of this  Debenture,  together  with all accrued  interest  thereon shall
become and be immediately  due and payable  without any declaration or other act
on the part of any Debenture holder.

              (c)  Neither  the holder of this  Debenture  nor the holder of any
other Series 2001-A  Debentures  shall  institute any suit or proceeding for the
enforcement  of the payment of  principal  or  interest  under any of the Series
2001-A  Debentures  unless  the  holders  of more than  fifty  percent  (50%) in
principal amount of all  then-outstanding  Series 2001-A Debentures join in such
suit or proceeding. Holders of more than fifty percent (50%) in principal amount
of the then outstanding Series 2001-A Debentures may direct the time, method and
place  of  conducting  any  proceeding  for  any  remedy  available  to  them or
exercising any power  conferred on them by the Debentures.  Notwithstanding  the
preceding  provisions of this  paragraph,  (i) any  Debenture  holder may file a
claim on their own behalf in any  proceeding to which  Borrower is subject under
the United States Bankruptcy Code or any other Debtor Relief Law; and (ii) if an
Event of  Default  arising  from the  failure of  Borrower  to pay  interest  or
principal  occurs  and  continues  for a  period  of  thirty  days  without  the
commencement  of any lawsuit or other  proceeding  by holders of more than fifty
percent  (50%)  in  principal  amount  of the  then  outstanding  Series  2001-A
Debentures,  any Debenture  holder may commence a lawsuit to enforce  payment of
principal and interest then due on the holder's  Debenture  (but not  accelerate
the  Maturity  Date of such  holder's  Debenture  or any  other  Series  2001- A
Debentures).

              (d) The Borrower  will furnish to any holder upon written  request
and  without  charge  a copy  of the  list of the  names  and  addresses  of the
registered  holders of Series 2001-A Debentures for a purpose reasonably related
to the interest of the holder as a Debenture holder.

         9.  Mutilated or Missing  Debentures.  In case any of the  certificates
evidencing the Debentures  shall be mutilated,  lost,  stolen or destroyed,  the
Borrower may in its  discretion  issue and deliver in exchange and  substitution
for  and  upon  cancellation  of the  mutilated  Debenture,  or in  lieu  of and
substitution  for the Debenture  lost,  stolen or destroyed,  a new Debenture of
like tenor,  but only upon receipt of evidence  reasonably  satisfactory  to the
Borrower of such loss,  theft or  destruction of such Debenture and an indemnity
or  bond,  if  requested,  also  reasonably  satisfactory  to the  Borrower.  An
applicant  for such a  substitute  Debenture  shall also  comply with such other
reasonable regulations and pay such other reasonable charges as the Borrower may
prescribe.

                                        4





         10. Law Governing. This Debenture shall be governed by and construed in
accordance  with the laws of the State of  California  applicable  to  contracts
entered into in the State of California, by residents of the State of California
and intended to be performed entirely within the State of California.

         11. Financial Covenants. Commencing on October 1, 2001 until such time,
subsequent  to the date of original  issue of this  Debenture,  as Borrower  (a)
obtains at least  $5,000,000 in additional cash in the aggregate  through one or
more sales of equity  securities  (including,  but not  limited  to, the sale of
capital  stock,  the sale or exercise of  options,  warrants or other  rights to
acquire  capital  stock,  and the conversion or exchange of any debt security or
evidence of indebtedness for capital stock), or (b) pays in full the outstanding
principal balance of, and all interest accrued on, the Series 2001-A Debentures,
Borrower's  total cash payments during any calendar quarter (ie. the three month
periods  ending March 31, June 30,  September 30, and December 31 of each year),
including,  without  limitation,  payment on account of  operating  expenses but
excluding  interest  payable on the Series 2001-A  Debentures,  shall not exceed
$450,000  in excess of all  revenues  collected  by Borrower in cash during such
calendar  quarter.  For the  purpose of this  Section,  revenues  shall  include
royalties,  license fees,  and other  proceeds from the sale or licensing of any
tangible or intangible property, but shall not include interest,  dividends, and
any monies borrowed or the proceeds from the issue or sale of any debt or equity
security.  Until such time as Borrower  pays in full the  outstanding  principal
balance of, and all interest accrued on, the Series 2001-A Debentures,  Borrower
shall not declare or pay any cash  dividend  on its  capital  stock or redeem or
repurchase any shares of its capital stock.

         12. Amendments, Supplements and Waiver.

              (a) Except as  provided  in the next  succeeding  paragraph,  this
Debenture and the other Series 2001-A  Debentures may be amended or supplemented
with the consent of the holders of at least a majority  in  principal  amount of
the Debentures then outstanding, and any existing default or compliance with any
provision of the Series 2001-A  Debentures,  including  this  Debenture,  may be
waived with the consent of the holders of a majority in principal  amount of the
then  outstanding  Series  2001-A  Debentures.  If any such  amendment or waiver
pertains to or affects some but not all of the Series  2001-A  Debentures,  such
consent need be obtained only from the holders of the Debentures so affected. If
any such  amendment or waiver would affect all of the Series  2001-A  Debentures
but would do so in a manner that differs  materially  between the  Debentures so
affected,  then the  consent of holders of a  majority  in  principal  amount of
Debenture  affected  in a like  manner  must be  obtained  to give effect to the
amendment or waiver.

              (b)  Without  the  written  consent of each  holder  affected,  an
amendment  or  waiver  may  not  (with  respect  to  any  Debenture  held  by  a
non-consenting  holder of Debentures) (i) reduce the principal  amount or change
the fixed maturity of any Debenture;  (ii) reduce the rate of or change the time
for  payment  of  interest  on any  Debenture;  (iii)  waive an Event of Default
arising  from the  failure of Borrower  to pay  principal  of or interest on the
Debentures (except a rescission of acceleration of the Debentures by the holders
of at least a majority in aggregate  principal  amount of the  Debentures  and a
waiver of the payment default that resulted from such acceleration); (iv) make

                                        5




any Debenture payable in money other than that stated in the Debentures;  or (v)
make any change in the provisions of the Debentures  relating to waivers of past
defaults or the rights of holders of Debentures to receive payments of principal
of or interest on the Debentures,  or make any change in the foregoing amendment
and waiver provisions.

              (c)  Notwithstanding  the  foregoing,  without  the consent of any
holder of Debentures, the Borrower may amend or supplement the Debentures to (i)
cure any ambiguity, defect or inconsistency,  (ii) to provide for uncertificated
Debentures  in  addition  to or in place of  certificated  Debentures,  (iii) to
provide  for  the  assumption  of  the  Borrower's  obligations  to  holders  of
Debentures  in the  case of a  merger  or  consolidation,  (iv) to  provide  any
collateral for the benefit of the holders of Debentures,  (v) to make any change
that  would  provide  any  additional  rights  or  benefits  to the  holders  of
Debentures  or that  does not  adversely  affect  the  legal  rights of any such
holder,  and (vi) to comply with the Trust  Indenture  Act of 1939,  if such law
becomes applicable to the Debentures.

         13. Release of Shareholders,  Officers and Directors. This Debenture is
the obligation of Borrower only, and no recourse shall be had for the payment of
this  Debenture  or the interest  thereon  against any  shareholder,  officer or
director of  Borrower,  either  directly or through  Borrower,  by virtue of any
statute for the  enforcement of any assessment or otherwise,  all such liability
of shareholders,  directors and officers as such being released by holder by the
acceptance of this Debenture.

         14. Fees and  Charges of  Attorneys  and Others.  In the event that the
holders  of  the  Series  2001-A  Debentures   employ  attorneys,   accountants,
appraisers,  consultants,  or other professional assistance,  in connection with
any of the following,  then, the reasonable amount of costs,  expenses, and fees
incurred by such Debenture  holders shall be payable on demand;  provided,  that
Borrower  shall not be  obligated  to pay the fees and expenses of more than one
firm of attorneys, accountants,  appraisers, consultants and other professionals
representing  all of the  Debenture  holders as a group.  Costs,  expenses,  and
reasonable fees of professionals  covered by this provision include such charges
for the following:

              (a) The preparation, modification, or renewal of the Series 2001-A
Debentures  and that  certain  Warrant  Agreement,  of even  date,  executed  by
Borrower in connection  with the issuance of common stock  purchase  warrants to
the persons to whom the 2001-A  Debentures were originally  issued and sold (the
"Warrant Agreement")..

              (b)  Any  litigation,   dispute,  proceeding  or  action,  whether
instituted by the Debenture holders,  Borrower, or any other person, relating to
the Series 2001-A Debentures,  including  representation of Debenture holders in
any bankruptcy,  insolvency,  or reorganization case or proceeding instituted by
or against Borrower,  and any attempt by Debenture holders to enforce any rights
against Borrower under the Series 2001-A Debentures;

                                        6



              (c) In the event of any controversy, claim, or dispute relating to
the  Series  2001-A  Debentures,  including  but not  limited  to any  action to
construe or enforce the terms of the Series 2001-A  Debentures,  the  prevailing
party shall be entitled to recover its reasonable costs,  expenses, and attorney
fees;

              (d) In the event of bankruptcy or insolvency  proceedings (whether
state or federal)  instituted by or against Borrower,  the Debenture holders may
recover all costs, expenses, and reasonable attorney fees incurred to protect or
defend their rights under the  Debentures,  and other  documents  underlying the
loan transactions whether such costs, expenses, and attorney fees be contractual
or  bankruptcy  related,  including  costs,  expenses,  and  attorney  fees  for
meetings,  sessions, matters, proceedings and litigation involving issues solely
distinct  to federal  bankruptcy  law,  rules and  proceedings  as well as other
federal and state litigation and proceedings; and

              (e) The preparation and filing of all reports required to be filed
by Debenture  holders  under the  Securities  Exchange Act of 1934,  as amended,
during the term of this Debenture in connection with the ownership, acquisition,
or disposition of the warrants, common shares, or other equity securities issued
by Borrower pursuant to the Warrant Agreement.

         15. Miscellaneous.

              (a) Borrower waives (i) presentment,  demand,  protest,  notice of
dishonor,  and all other notices; (ii) any release or discharge arising from any
extension of time,  or discharge of a prior party;  and (iii) any other cause of
release or  discharge  other  than  actual  payment in full of all  indebtedness
evidenced by or arising under this Debenture.

              (b) The rights and  remedies  of the  holder as  provided  in this
Debenture and in law or equity shall be cumulative  and  concurrent,  and may be
pursued  singularly,  successively,  or together at the sole  discretion  of the
holder  subject to the provisions of Section 8, and may be exercised as often as
occasion  therefor  shall  occur;  and the failure to exercise any such right or
remedy shall in no event be construed as a waiver or a release of any such right
or remedy.

              (c)  The  terms,  covenants,  and  conditions  contained  in  this
Debenture   shall  be  binding  upon  the  heirs,   executors,   administrators,
successors,  and  assigns of  Borrower,  and shall  inure to the  benefit of the
heirs, executors, administrators, successors and assigns of holder.

              (d) If any provisions of this Debenture would require the Borrower
to pay interest on the indebtedness evidenced by or arising under this Debenture
at a rate exceeding the highest rate

                                        7





allowable by applicable  law,  Borrower  shall  instead pay interest  under this
Debenture at the highest rate permitted by applicable law.

         IN WITNESS  WHEREOF,  Borrower has signed and sealed this  Debenture on
the date first set forth above.




                                     BIOTIME, INC.


                                     By:_______________________________


                                     Title: ___________________________



                                     By:_______________________________


                                     Title: ___________________________



                                        8