<PAGE>   1
     THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, OR THE SECURITIES LAWS OF ANY STATE AND MAY NOT BE SOLD OR OTHERWISE
TRANSFERRED IN THE ABSENCE OF EFFECTIVE REGISTRATION STATEMENTS UNDER SUCH ACT
AND STATE SECURITIES LAWS, OR AN OPINION OF COUNSEL REASONABLY ACCEPTABLE TO THE
DEBTOR THAT SUCH REGISTRATION IS NOT REQUIRED.

                                 DYNAGEN, INC.

                               SUBORDINATED NOTE

$155,000.00                                                December 19, 1997


     FOR VALUE RECEIVED, DYNAGEN, INC., a Delaware corporation (the "DEBTOR"),
hereby promises to pay on or before January 18, 1998, to the order of Sovereign
Partners LP (the "LENDER"), the principal sum of ONE HUNDRED THOUSAND FIFTY-FIVE
DOLLARS ($155,000.00) or such lesser principal amount then outstanding, together
with all accrued and unpaid interest thereon. Interest on the principal amount
of this Note will accrue from and including the date hereof until and including
the date such principal amount is paid, at a rate equal to seven percent (7.0%)
per annum. Principal and interest shall be payable in lawful money of the United
States of America, in immediately available funds, at the principal office of
the Lender or at such other place as the legal holder may designate from time to
time in writing to the Debtor. Interest shall be computed on the basis of a
360-day year and a 30-day month.

     At the option of the Lender, the principal amount of this Note may be
satisfied in full by exchanging this Note on January 18, 1998 for conversion
into shares of the Debtor's Series A Preferred Stock, $.01 par value per share.

     This Note is being issued in connection with a certain Bridge Financing
Purchase Agreement dated the date hereof. The outstanding balance of this Note
shall be immediately due and payable upon an Event of Default as defined in such
Bridge Financing Purchase Agreement.

     The Lender and any holder of this Note acknowledge and agree that the
principal and interest on this Note are unsecured and the rights and obligations
represented by this Note are subordinated in right of payment to secured
indebtedness of the Debtor for money borrowed from commercial bank lenders or
other institutional lenders or other holders of Senior Debt, whether now
existing or hereafter arising, as further defined in the Bridge Financing
Purchase Agreement.

     This Note applies to, inures to the benefit of, and binds the successors
and assigns of the parties hereto. This Note is made under and shall be governed
by and construed in accordance with the internal laws of, and enforced by the
courts located within, the Commonwealth of Massachusetts.  


<PAGE>   2


                                     - 2 -


     IN  WITNESS WHEREOF, the Debtor has executed this Note as an instrument
under seal as of the date first written above.

                                       DYNAGEN, INC.

     
                                       By:____________________________ 


                                       Title:_________________________

ATTEST:___________________________


Title:____________________________