Exhibit 10d


         NEITHER THIS DEBENTURE NOR THE SECURITIES  INTO WHICH THIS DEBENTURE IS
CONVERTIBLE HAVE BEEN REGISTERED WITH THE SECURITIES AND EXCHANGE  COMMISSION OR
THE  SECURITIES  COMMISSION  OF ANY STATE IN  RELIANCE  UPON AN  EXEMPTION  FROM
REGISTRATION  UNDER THE UNITED  STATES  SECURITIES  ACT OF 1933, AS AMENDED (THE
"SECURITIES ACT"), AND, ACCORDINGLY,  MAY NOT BE OFFERED OR SOLD EXCEPT PURSUANT
TO AN EFFECTIVE  REGISTRATION  STATEMENT UNDER THE SECURITIES ACT OR PURSUANT TO
AN  AVAILABLE   EXEMPTION  FROM,  OR  IN  A  TRANSACTION  NOT  SUBJECT  TO,  THE
REGISTRATION   REQUIREMENTS  OF  THE  SECURITIES  ACT  AND  IN  ACCORDANCE  WITH
APPLICABLE STATE SECURITIES LAWS.

         THIS  DEBENTURE  IS SUBJECT TO CERTAIN  RESTRICTIONS  ON  TRANSFER  AND
CONVERSION SET FORTH IN A CONVERTIBLE DEBENTURE PURCHASE AGREEMENT,  DATED AS OF
JULY 17,  1997,  AS AMENDED FROM TIME TO TIME,  EXECUTED BY THE ORIGINAL  HOLDER
HEREOF.  A COPY OF THAT  AGREEMENT  IS ON FILE AT THE  PRINCIPAL  OFFICE  OF PLC
SYSTEMS INC.

No. [   ]                                                           U.S. $[    ]


                                PLC SYSTEMS INC.
                     5% CONVERTIBLE DEBENTURE DUE [  ], 2002

         THIS DEBENTURE is one of a duly  authorized  issue of debentures of PLC
Systems Inc. a  corporation  organized  and  existing  under the laws of British
Columbia,  Canada,  having a  principal  place  of  business  at 10 Forge  Park,
Franklin, MA 02038 (the Company ), designated as its 5% Convertible  Debentures,
due [ ],  2002,  in an  aggregate  principal  amount of up to  $20,150,000  (the
"Debentures").

         FOR VALUE RECEIVED,  the Company  promises to pay to [ ], or registered
assigns (the Holder ), the principal sum of [ ] dollars ($[ ]), on or prior to [
], 2002 or such  earlier  date as the  Debentures  are  required to be repaid as
provided hereunder (the Maturity Date ) and to pay interest to the Holder on the
principal sum at the rate of 5% per annum,  payable upon  conversion as provided
hereunder,  or on the Maturity  Date if not earlier  converted.  Interest  shall
accrue daily  commencing  on the  Original  Issue Date (as defined in Section 6)
until payment in full of the principal sum, together with all accrued and unpaid
interest  and other  amounts  which may  become  due  hereunder,  has been made.
Interest  shall be  calculated on the basis of a 360-day year and for the actual
number of days elapsed.  Interest  hereunder will be paid to the person in whose
name this Debenture (or one or more predecessor Debentures) is registered on the
records of the Company  regarding  registration  and transfers of the Debentures
(the  Debenture  Register ) on the  Conversion  Date (as  defined  below) or the
Maturity  Date,  as the  case may be;  provided,  however,  that  the  Company's
obligation to a transferee of this Debenture arises 








only if such transfer,  sale or other disposition is made in accordance with the
terms and conditions hereof and of the Convertible Debenture Purchase Agreement,
dated  as of July  17,  1997,  as  amended  from  time to  time  (the  "Purchase
Agreement"),  executed by the original Holder.  All overdue,  accrued and unpaid
interest and other amounts due hereunder  shall bear interest at the rate of 15%
per annum from the day of  conversion  hereunder or the Maturity Date or earlier
date on which this  Debenture is  accelerated  through and including the date of
payment.  The  principal of, and interest on, and other amounts owing in respect
of this  Debenture  are payable in such coin or currency of the United States of
America  as at the time of payment  is legal  tender  for  payment of public and
private  debts,  at the address of the Holder last  appearing  on the  Debenture
Register,  except that  principal  paid on [ ], 2002 and interest due  hereunder
may, at the Company's  option, be paid in shares of the Common Stock (as defined
in Section 6)  calculated  based upon the average Per Share Market Value for the
five (5) Trading Days immediately  preceding the Conversion Date,  Maturity Date
or the date upon which interest shall cease to accrue, as provided below, as the
case may be. All amounts due hereunder  shall be paid in cash or in Common Stock
as provided herein.  Notwithstanding  anything to the contrary contained herein,
the Company may not issue  shares of the Common  Stock in payment of interest on
the  principal  amount if: (i) the number of shares of Common  Stock at the time
authorized, unissued and unreserved for all purposes, or held as treasury stock,
is insufficient to pay interest  hereunder in shares of Common Stock;  (ii) such
shares are not  registered  for resale  pursuant  to an  effective  registration
statement  that  names  the  recipient  of such  interest  shares  as a  selling
stockholder  thereunder and may not be sold without volume restrictions pursuant
to Rule 144  promulgated  under the  Securities  Act of 1933,  as  amended  (the
"Securities Act"), as determined by counsel to the Company pursuant to a written
opinion letter, addressed to the Holder, in form and substance acceptable to the
Holder;  (iii) such shares are not listed on the American Stock Exchange (or the
Nasdaq National  Market,  Nasdaq National Market or The New York Stock Exchange)
and any other exchange on which the Common Stock is then listed for trading;  or
(iv)  the  issuance  of  such  shares  would  result  in the  recipient  thereof
beneficially  owning  more than 4.999% of the issued and  outstanding  shares of
Common Stock as determined in  accordance  with Rule 13d-3 under the  Securities
Exchange  Act of 1934,  as amended.  Payment of interest  on the  Debentures  in
shares of Common Stock is also subject to the provisions of Section 4(a)(ii).  A
transfer of the right to receive  principal  and interest  under this  Debenture
shall be  transferable  only  through  an  appropriate  entry  in the  Debenture
Register as provided herein.  Notwithstanding anything to the contrary contained
herein,  interest hereunder will cease to accrue if the average Per Share Market
Value for any  consecutive 30 Trading Day period  commencing  after the Original
Issue Date exceeds the Per Share Market Value on the Original Issue Date by more
than 50%. In such event,  interest  accrued  through  such date shall be due and
payable.

         This Debenture is subject to the following additional provisions:

                  Section 1. The Debentures are issuable in denominations of Two
Hundred  Fifty  Thousand  Dollars  ($250,000)  and  integral  multiples of Fifty
Thousand  Dollars  ($50,000) in excess thereof.  The Debentures are exchangeable
for an equal  aggregate  amount of preferred  shares of the Company  which shall
have  identical  terms to the  terms  of this  


                                       2






Debenture,  at any time prior to Maturity  at the option of the Company  upon 30
Trading Days notice to the Holders,  provided,  that no such Notice may be given
by the Company at any time after  notice of a default  which if not timely cured
hereunder would constitute an Event of Default hereunder. No service charge will
be made for such registration of transfer or exchange.

                  Section 2. This  Debenture has been issued  subject to certain
investment  representations  of the  original  Holders set forth in the Purchase
Agreement  and may be  transferred  or  exchanged  only in  compliance  with the
Purchase Agreement. Prior to due presentment to the Company for transfer of this
Debenture,  the  Company  and any agent of the  Company  may treat the person in
whose name this  Debenture is duly  registered on the Debenture  Register as the
owner hereof for the purpose of receiving payment as herein provided and for all
other  purposes,  whether or not this  Debenture  is  overdue,  and  neither the
Company nor any such agent shall be affected by notice to the contrary.

                  Section 3.   Events of Default.

         "Event  of  Default",  wherever  used  herein,  means  any  one  of the
following  events  (whatever  the reason and  whether it shall be  voluntary  or
involuntary or effected by operation of law or pursuant to any judgment,  decree
or order of any court, or any order,  rule or regulation of any Federal,  state,
local or foreign administrative or governmental body):

                  (a) any default in the payment of the principal  of,  interest
         on or  liquidated  damages in respect of, this  Debenture,  free of any
         claim of  subordination,  as and when the  same  shall  become  due and
         payable on the Conversion  Date or the Maturity Date or by acceleration
         or otherwise;

                  (b) the Company  shall fail to observe or perform any warranty
         contained in, or any  representation of the Company shall prove to have
         been incorrect  when given under,  or the Company shall fail to observe
         or perform in any material  respect any covenant or agreement under, or
         otherwise commit any breach of, this Debenture, the Purchase Agreement,
         any Warrant (as defined in the Purchase  Agreement) or the Registration
         Rights  Agreement (as defined in Section 6), and such failure or breach
         shall not have  been  remedied  within 10 days  after the date on which
         notice of such failure or breach shall have been given;

                  (c) the Company or any of its subsidiaries shall commence,  or
         there shall be commenced  against the Company or any such  subsidiary a
         case  under any  applicable  bankruptcy  or  insolvency  laws as now or
         hereafter in effect or any successor thereto,  or the Company commences
         any other proceeding under any reorganization,  arrangement, adjustment
         of debt, relief of debtors,  dissolution,  insolvency or liquidation or
         similar  law of any  jurisdiction  whether now or  hereafter  in effect
         relating to the Company or any subsidiary thereof or there is commenced
         against the  Company or any  subsidiary  thereof  any such  bankruptcy,
         insolvency or other proceeding  which remains  undismissed for a period
         of 60 days;  or the Company or any  subsidiary  thereof is  

 
                                        3






         adjudicated  insolvent  or  bankrupt;  or any  order of relief or other
         order approving any such case or proceeding is entered;  or the Company
         or any subsidiary  thereof  suffers any appointment of any custodian or
         the like for it or any substantial part of its property which continues
         undischarged or unstayed for a period of 60 days; or the Company or any
         subsidiary  thereof  makes a  general  assignment  for the  benefit  of
         creditors;  or the Company shall fail to pay, or shall state that it is
         unable to pay, or shall be unable to pay,  its debts  generally as they
         become  due;  or the  Company or any  subsidiary  thereof  shall call a
         meeting of its  creditors  with a view to  arranging a  composition  or
         adjustment of its debts; or the Company or any subsidiary thereof shall
         by any act or failure to act  indicate  its consent to,  approval of or
         acquiescence in any of the foregoing;  or any corporate or other action
         is taken by the  Company or any  subsidiary  thereof for the purpose of
         effecting any of the foregoing;

                  (d) the Company shall default in any of its obligations  under
         any mortgage,  credit agreement or other facility,  indenture agreement
         or other  instrument  under which there is  indebtedness of the Company
         exceeding one hundred thousand dollars  ($100,000) secured by assets of
         the Company (or any Subsidiary thereon),  whether such indebtedness now
         exists or shall  hereafter be created and such default  shall result in
         such  indebtedness  becoming or being declared due and payable prior to
         the date on which it would otherwise become due and payable;

                  (e) the Common Stock shall be delisted from the American Stock
         Exchange or any other national  securities  exchange or market on which
         such  Common  Stock is listed for  trading or  suspended  from  trading
         thereon without being relisted or having such suspension lifted, as the
         case may be, within three Trading Days; or

                  (f)  the   Company   shall  be  a  party  to  any   merger  or
         consolidation  pursuant to which the Company shall not be the surviving
         entity or shall  dispose of all or  substantially  all of its assets in
         one or more transactions, or shall redeem more than a de minimis number
         of shares of Common Stock (other than an  aggregate  of  $1,000,000  of
         shares of Common Stock which may be  repurchased  from  employees  upon
         their  termination  of  employment  with  the  Company,  calculated  by
         reference  to the market  price of the Common  Stock at such time,  and
         other than redemptions of Underlying Shares (as defined in Section 6)).

If during the time that any portion of this Debenture remains  outstanding,  any
Event of Default  occurs  and is  continuing,  and in every such case,  then the
Holders may, by notice to the Company, declare the full principal amount of this
Debenture, together with all accrued but unpaid interest and other amounts owing
hereunder, to the date of acceleration,  to be, plus the "Adjustment Amount" (as
defined in Section 6)  whereupon  the same  shall  become,  immediately  due and
payable in cash  (notwithstanding  anything  herein  contained to the  contrary)
without presentment,  demand,  protest or other notice of any kind, all of which
are waived by the  Company,  notwithstanding  anything  herein  contained to the
contrary,  and the Holder may  immediately  and without  expiration of any grace
period  enforce any and all of its rights and remedies  hereunder  and all other
remedies available to it under applicable law. 


                                       4






Such  declaration  may be rescinded  and annulled by Holder at any time prior to
payment  hereunder.  No such rescission or annulment shall affect any subsequent
Event of Default or impair any right consequent thereon.

                  Section 4.   Conversion.

                  (a) (i) This Debenture shall be convertible into shares of the
Common  Stock  (subject  to  reduction  pursuant to Section  4(a)(ii)  below and
Section 3.7 of the Purchase  Agreement,  at the option of the Holder in whole or
in part at any time and from time to time  after  the  Original  Issue  Date and
prior to the close of business on the  Maturity  Date.  The Holder  shall effect
conversions by  surrendering  the  Debentures  (or such portions  thereof) to be
converted,  together  with the form of  conversion  notice  attached  hereto  as
Exhibit A (the "Conversion Notice") to the Company. Each Conversion Notice shall
specify the principal  amount of  Debentures  to be converted  (which may not be
less than $100,000 or such less principal amount of Debentures than held by such
Holder) and the date on which such conversion is to be effected,  which date may
not be prior to the date such Conversion Notice is deemed to have been delivered
pursuant to Section  4(h) (the  "Conversion  Date").  If no  Conversion  Date is
specified in a Conversion Notice, the Conversion Date shall be the date that the
Conversion  Notice is deemed  delivered  pursuant  to Section  4(h).  Subject to
Sections  4(a)(ii)  and 4(b) hereof and Section 3.7 of the  Purchase  Agreement,
each  Conversion  Notice,  once given,  shall be  irrevocable.  If the Holder is
converting less than all of the principal amount represented by the Debenture(s)
tendered by the Holder with the Conversion Notice, or if a conversion  hereunder
cannot  be  effected  in full for any  reason,  the  Company  shall  honor  such
conversion to the extent  permissible  hereunder and shall  promptly  deliver to
such Holder (in the manner and within the time set forth in Section  5(b)) a new
Debenture for such principal amount as has not been converted.

                      (ii) Certain  Regulatory  Approval.  If on the  Conversion
Date  applicable  to any  conversion,  (A) the Common  Stock is then  listed for
trading on the American Stock Exchange or Nasdaq National Market or if the rules
of the  Nasdaq  Stock  Market  are  hereafter  amended  to extend  Rule  4460(i)
promulgated  thereby (or any successor or replacement  provision thereof) to the
Nasdaq  SmallCap  Market and the Common Stock is then listed for trading on such
market or  exchange,  (B) the  Conversion  Price then in effect is such that the
aggregate  number of shares of the Common Stock that would then be issuable upon
conversion of the entire  outstanding  principal amount of Debentures,  together
with any  shares of the  Common  Stock  previously  issued  upon  conversion  of
Debentures and in respect of payment of interest hereunder would equal or exceed
20% of the  number of shares of the Common  Stock  outstanding  on the  Original
Issue Date (the  "Issuable  Maximum"),  and (C) the Company  has not  previously
obtained the  Stockholder  Approval (as defined  below),  then the Company shall
issue to the Holder  requesting such  conversion the Issuable  Maximum and, with
respect  to any  shares of the  Common  Stock  that  otherwise  would  have been
issuable  to such  holder in  respect  of the  Conversion  Notice at issue or in
respect of payment of interest hereunder in excess of the Issuable Maximum,  the
Holder shall have the option to require the Company to either (1) as promptly as
possible, but in no event later than 90 days after such Conversion Date, convene
a meeting of the holders of the Common Stock and use its  


                                       5







reasonable efforts to obtain the Stockholder  Approval or (2) prepay, from funds
legally  available  therefor at the time of such prepayment,  the balance of the
principal  amount of the Debentures  subject to such  Conversion  Notice and the
remaining  aggregate principal amount of Debentures then outstanding and held by
such  Holder  at a  price  equal  to the  sum of (A)  the  principal  amount  of
Debentures then outstanding and held by such Holder multiplied by the product of
(i) the average Per Share Market Value for the five (5) Trading Days immediately
preceding  (1) the  Conversion  Date or (2) the date of  payment  in full by the
Company of such  prepayment  price,  whichever  is greater,  divided by (ii) the
Conversion  Price  calculated  on the  Conversion  Date plus (B) all accrued and
unpaid  interest  and other  amounts  then due in  respect  of such  Debentures;
provided,  however,  that if the Holder has  requested  that the Company  obtain
Stockholder  Approval  under  paragraph  (1) above and the Company fails for any
reason to obtain such  Stockholder  Approval within the time period set forth in
(1) above,  the Company  shall be obligated to prepay  Debentures  in accordance
with the  provisions  of  paragraph  (2)  above,  and in such case the  interest
contemplated  by the immediately  succeeding  sentence shall be deemed to accrue
from the  Conversion  Date. If the Holder has requested  that the Company prepay
Debentures  pursuant to this Section and the Company fails for any reason to pay
the  prepayment  price under (2) above  within  seven days after the  Conversion
Date,  the Company will pay interest on such  prepayment  price at a rate of 15%
per annum to the converting Holder,  accruing from the Conversion Date until the
prepayment  price plus any accrued  interest thereon is paid in full. The entire
prepayment  price,   including   interest  thereon,   shall  be  paid  in  cash.
"Stockholder  Approval" means the approval by a majority of the total votes cast
on the proposal,  in person or by proxy, at a meeting of the stockholders of the
Company held in accordance with the Company's  Certificate of Incorporation  and
by-laws,  of the issuance by the Company of shares of the Common Stock exceeding
the Issuable  Maximum as a consequence of the conversion of Debentures  into the
Common Stock at a price less than the greater of the book or market value on the
Original  Issue Date as and to the extent  required  pursuant to Rule 713 of the
American  Stock  Exchange  or Rule  4460(i) of the Nasdaq  Stock  Market (or any
successor or replacement provision thereof), as applicable.

                      (iii) Maximum Conversion Shares.  Notwithstanding anything
to the contrary set forth herein, the Company shall not be obligated to issue in
excess of 2,500,000 shares of Common Stock upon conversion of Debentures,  which
shares shall  constitute  payment in full of all accrued interest and payment of
principal due on the expiration date of such  Debentures,  which number shall be
subject  to  adjustment  pursuant  to  Section 4 (but not in the event  that the
Debentures  are  accelerated  in  accordance   herewith).   Notwithstanding  the
foregoing,  should  the  Company  receive  either  (i) a  recommendation  of new
approval  of its  Pre-Market  Application  for its Heart  Laser  System from the
Circulatory  Systems  Advisory  Panel (the  "Panel")  of the U.S.  Food and Drug
Administration   (the  "FDA"),   or  (ii)  should  the  Company  not  receive  a
recommendation of approval of its Pre-Market  Application for Heart Laser System
from the Panel or the FDA by August 14, 1998,  then the maximum number of shares
of Common Stock which the Company shall be obligated to issue upon conversion of
the Debentures  shall be increased  from  2,500,000 to 3,000,000.  In accordance
with the  Purchase  Agreement  such number of shares shall be available on a pro
rata basis based upon the pro rata purchase price for the Debentures paid by the
Original  Holders of  Debentures.  Shares of Common  Stock  issued in respect of
penalties and liquidated


                                       6






damages  hereunder  shall not count towards the share limits  referenced in this
paragraph and shall be paid in cash unless otherwise agreed to by the Holders.

                  (b) Not later than  three  Trading  Days after the  Conversion
Date, the Company will deliver to the Holder (i) a certificate  or  certificates
which shall be free of restrictive legends and trading  restrictions (other than
those required by Section 3.1 of the Purchase Agreement) representing the number
of shares of the Common Stock being  acquired upon the  conversion of Debentures
(subject to reduction pursuant to Section 4(a)(ii) hereof and Section 3.7 of the
Purchase  Agreement),  (ii)  Debentures  in a  principal  amount  equal  to  the
principal  amount of Debentures not converted;  (iii) a bank check in the amount
of all  accrued and unpaid  interest  (if the Company has elected to pay accrued
interest  in cash),  together  with all other  amounts  then due and  payable in
accordance  with the  terms  hereof,  in  respect  of  Debentures  tendered  for
conversion and (iv) if the Company has elected to pay accrued interest in shares
of the Common Stock,  certificates,  which shall be free of restrictive  legends
and trading  restrictions (other than those required by the Purchase Agreement),
representing  such number of shares of the Common Stock as equals such  interest
divided  by the  average  Per  Share  Market  Value  for the five  Trading  Days
immediately  preceding the Conversion Date; provided,  however, that the Company
shall not be obligated to issue certificates evidencing the shares of the Common
Stock  issuable  upon  conversion of the  principal  amount of Debentures  until
Debentures  are either  delivered for  conversion to the Company or any transfer
agent for the Debentures or the Common Stock, or the Holder notifies the Company
that such  Debenture has been lost,  stolen or destroyed and provides a bond (or
other adequate security) reasonably satisfactory to the Company to indemnify the
Company from any loss incurred by it in connection  therewith (in which case the
Company  shall issue a  replacement  Debenture in like  principal  amount).  The
Company shall,  upon request of the Holder,  use its best efforts to deliver any
certificate or  certificates  required to be delivered by the Company under this
Section  electronically  through the  Depository  Trust  Corporation  or another
established clearing corporation performing similar functions. If in the case of
any Conversion  Notice such certificate or certificates,  including for purposes
hereof,  any shares of the Common Stock to be issued on the  Conversion  Date on
account of accrued but unpaid  interest  hereunder,  are not  delivered to or as
directed by the applicable  Holder by the third Trading Day after the Conversion
Date,  the Holder shall be entitled by written notice to the Company at any time
on or before its receipt of such  certificate  or  certificates  thereafter,  to
rescind such conversion, in which event the Company shall immediately return the
Debentures  tendered  for  conversion.  If the  Company  fails to deliver to the
Holder such certificate or certificates pursuant to this Section,  including for
purposes  hereof,  any shares of the Common Stock to be issued on the Conversion
Date on account of accrued  but unpaid  interest  hereunder,  prior to the fifth
Trading Day after the Conversion Date, the Company shall pay to such Holder,  in
cash, as liquidated damages and not as a penalty, $1,500 for each day after such
fifth Trading Day until such certificates are delivered. If the Company fails to
deliver to the Holder such certificate or certificates  pursuant to this Section
prior to the 15th day after the  Conversion  Date,  the  Company  shall,  at the
Holder's option (i) prepay, from funds legally available therefor at the time of
such  prepayment,  the aggregate of the principal amount of Debentures then held
by such Holder, as requested by such Holder, and (ii) pay all accrued but unpaid
interest on account of the Debentures for which the Company shall have failed to
issue the Common 

                                       7






Stock  certificates  hereunder,  in cash. The prepayment price shall be equal to
the sum of (A) the  aggregate  of all  accrued  but  unpaid  interest  and other
non-principal  amounts then payable in respect of all  Debentures  to be prepaid
hereunder and for which prepayment hereunder is demanded, plus (B) the aggregate
of the principal amount of Debentures then held by such Holder multiplied by (1)
the average Per Share  Market  Value for the five (5) Trading  Days  immediately
preceding  (x) the  Conversion  Date or (y) the date of  payment  in full by the
Company of such  prepayment  price,  whichever  is greater,  divided by, (2) the
Conversion  Price calculated on the Conversion Date. If the Holder has requested
that the Company  prepay  Debentures  pursuant  to this  Section and the Company
fails for any reason to pay the  prepayment  price  within seven days after such
notice is deemed  delivered  pursuant  to Section  4(h),  the  Company  will pay
interest  on the  prepayment  price at a rate of 15% per annum,  in cash to such
Holder,  accruing  from such  seventh  day until  the  prepayment  price and any
accrued interest thereon is paid in full.

                  (c) (i) The  conversion  price  (the  "Conversion  Price")  in
effect on any Conversion Date shall be the lesser of (a) 120% of the average Per
Share Market Value of the Common Stock for the ten (10) Trading Days immediately
preceding the Original Issue Date (the "Initial  Conversion  Price") and (b) the
average  of the five (5)  consecutive  lowest  Per Share  Market  Values for the
thirty (30) Trading Days  immediately  preceding the Conversion Date (which such
thirty  (30)  Trading  Day  period  must  commence  on or the issue date of this
Debenture);  provided that, (a) if the  registration  statement  registering the
resale of the shares of Common Stock issuable upon  conversion of the Debentures
and  payment  of  interest  thereunder  and  naming  the  Holder  as  a  Selling
Stockholder  thereunder (the "Underlying Shares Registration  Statement") is not
filed on or prior to the 30th day  after the  Original  Issue  Date,  or (b) the
Company  fails  to  file  with  the  Securities  and  Exchange  Commission  (the
"Commission")  a  request  for  acceleration  in  accordance  with  Rule  12d1-2
promulgated under the Securities  Exchange Act of 1934, as amended,  within five
(5) days of the  date  that the  Company  is  notified  (orally  or in  writing,
whichever is earlier) by the Commission that an Underlying  Shares  Registration
Statement  will not be "reviewed" or is not subject to further review or comment
by the Commission, or (c) if the Underlying Shares Registration Statement is not
declared  effective  by the  Commission  on or prior to the 90th day  after  the
Original Issue Date, or (d) if such Underlying Shares Registration  Statement is
filed with and declared  effective by the Commission but thereafter ceases to be
effective  as to all  Registrable  Securities  (as such term is  defined  in the
Registration  Rights  Agreement)  at any  time  prior to the  expiration  of the
"Effectiveness  Period"  (as such term as  defined  in the  Registration  Rights
Agreement),   without  being  succeeded  by  a  subsequent   Underlying   Shares
Registration  Statement  filed with and  declared  effective  by the  Commission
within 10  Business  Days (as  defined in  Section  6), or (e) if trading in the
Common Stock shall be suspended for any reason for more than three Trading Days,
or (f) if the  conversion  rights of the  Holders of  Debentures  hereunder  are
suspended for any reason (any such failure being  referred to as an "Event," and
for purposes of clauses (a), (c) and (f) the date on which such Event occurs, or
for  purposes  of  clause  (b) the date on which  such  five (5) days  period is
exceeded,  or for  purposes  of  clause  (d) the  date  which  such 10  Business
Day-period  is  exceeded,  or for  purposes of clause (e) the date on which such
three Trading Day period is exceeded,  being referred to as "Event  Date"),  the
Conversion  Price shall be  decreased by 2.5% on the Event Date and each monthly


                                        8








anniversary  thereof until the earlier to occur of the second month  anniversary
after the Event Date and such time as the applicable  Event is cured (i.e.,  the
Conversion  Price  would  decrease by 2.5% as of the Event Date and 5% as of the
one  month  anniversary  of  such  Event  Date).  Commencing  the  second  month
anniversary  after  the  Event  Date,  at the  option  of each  Holder  for each
applicable  monthly  period either (a) the Company shall pay to the Holders 2.5%
of the product of the principal  amount of outstanding  Debentures  (each Holder
being  entitled  to receive  such  portion of such amount as equals its pro rata
portion of Debentures  then  outstanding),  in cash or (b) the Conversion  Price
shall be  decreased by 2.5% for each  additional  such month (to be effective in
full on the monthly applicable Event Date) as liquidated  damages,  and not as a
penalty on the first day of each monthly anniversary of the Event Date in either
case until  such time as the  applicable  Event is cured.  Any  decrease  in the
Conversion Price pursuant to this Section shall remain in effect notwithstanding
the fact that the Event  causing such decrease has been  subsequently  cured and
further  monthly  decreases have ceased.  The provisions of this Section are not
exclusive  and  shall  in no way  limit  the  Company's  obligations  under  the
Registration Rights Agreement.

                      (iii) If the Company, at any time while any Debentures are
outstanding,  (a) shall pay a stock dividend or otherwise make a distribution or
distributions  on  shares  of its  Common  Stock or any  other  equity or equity
equivalent  securities  payable in shares of the  Common  Stock,  (b)  subdivide
outstanding  shares of the  Common  Stock into a larger  number of  shares,  (c)
combine  outstanding shares of the Common Stock into a smaller number of shares,
or (d) issue by  reclassification  of shares of the  Common  Stock any shares of
capital stock of the Company,  the Initial  Conversion Price shall be multiplied
by a fraction of which the numerator shall be the number of shares of the Common
Stock (excluding  treasury shares, if any) outstanding  before such event and of
which  the  denominator  shall be the  number  of  shares  of the  Common  Stock
outstanding  after such event.  Any  adjustment  made  pursuant to this  Section
4(c)(ii)  shall  become  effective  immediately  after the  record  date for the
determination of stockholders  entitled to receive such dividend or distribution
and shall become effective immediately after the effective date in the case of a
subdivision, combination or re-classification.

                      (iv) If the Company,  at any time while any Debentures are
outstanding,  shall issue  rights or warrants to all holders of the Common Stock
(and not to Holders of  Debentures)  entitling them to subscribe for or purchase
shares of the Common  Stock at a price per share less than the Per Share  Market
Value of the Common  Stock at the  record  date  mentioned  below,  the  Initial
Conversion  Price shall be  multiplied by a fraction,  of which the  denominator
shall be the number of shares of the Common Stock (excluding treasury shares, if
any)  outstanding  on the date of issuance  of such rights or warrants  plus the
number of  additional  shares of the Common Stock  offered for  subscription  or
purchase, and of which the numerator shall be the number of shares of the Common
Stock (excluding treasury shares, if any) outstanding on the date of issuance of
such rights or warrants plus the number of shares which the  aggregate  offering
price of the total number of shares so offered would  purchase at such Per Share
Market Value. Such adjustment shall be made whenever such rights or warrants are
issued,  and shall become  effective  immediately  after the record date for the
determination  of  stockholders  entitled to receive  such  rights or  warrants.
However,  upon the 


                                       9







expiration  of any right or warrant to purchase  shares of the Common  Stock the
issuance of which  resulted in an  adjustment  in the Initial  Conversion  Price
pursuant to this Section  4(c)(iii),  if any such right or warrant  shall expire
and  shall  not  have  been  exercised,   the  Initial  Conversion  Price  shall
immediately  upon such expiration be recomputed and effective  immediately  upon
such  expiration  be  increased  to the  price  which it would  have  been  (but
reflecting any other  adjustments in the Initial  Conversion Price made pursuant
to the  provisions  of this  Section  4 after  the  issuance  of such  rights or
warrants)  had the  adjustment  of the  Initial  Conversion  Price made upon the
issuance  of such  rights or  warrants  been made on the basis of  offering  for
subscription or purchase only that number of shares of the Common Stock actually
purchased upon the exercise of such rights or warrants actually exercised.

                      (v) If the  Company,  at any  time  while  Debentures  are
outstanding,  shall  distribute  to all holders of the Common  Stock (and not to
Holders of  Debentures)  evidences  of its  indebtedness  or assets or rights or
warrants to subscribe for or purchase any security  (excluding those referred to
in  Sections  4(c)(ii)  and (iii)  above),  then in each  such case the  Initial
Conversion Price at which  Debentures  shall thereafter be convertible  shall be
determined by multiplying  the Initial  Conversion  Price in effect  immediately
prior to the record date fixed for  determination  of  stockholders  entitled to
receive such  distribution by a fraction of which the  denominator  shall be the
Per Share  Market  Value of the Common  Stock  determined  as of the record date
mentioned above, and of which the numerator shall be such Per Share Market Value
of the Common  Stock on such record date less the then fair market value at such
record  date of the  portion  of such  assets or  evidence  of  indebtedness  so
distributed  applicable  to  one  outstanding  share  of  the  Common  Stock  as
determined by the Board of Directors in good faith;  provided,  however, that in
the event of a distribution exceeding ten percent (10%) of the net assets of the
Company,  such fair market value shall be determined by a nationally  recognized
or major  regional  investment  banking  firm or firm of  independent  certified
public accountants of recognized  standing (which may be the firm that regularly
examines the financial  statements of the Company) (an "Appraiser")  selected in
good  faith  by the  holders  of a  majority  in  interest  of  Debentures  then
outstanding;  and  provided,  further,  that the Company,  after  receipt of the
determination  by such  Appraiser  shall have the right to select an  additional
Appraiser,  in good faith, in which case the fair market value shall be equal to
the average of the  determinations  by each such  Appraiser.  In either case the
adjustments  shall be  described  in a  statement  provided  to the  holders  of
Debentures of the portion of assets or evidences of  indebtedness so distributed
or such  subscription  rights  applicable to one share of the Common Stock. Such
adjustment shall be made whenever any such distribution is made and shall become
effective immediately after the record date mentioned above.

                      (vi) All  calculations  under this Section 4 shall be made
to the nearest cent or the nearest 1/100th of a share, as the case may be.

                      (vii)  Whenever the Initial  Conversion  Price is adjusted
pursuant to Section  4(c)(ii),(iii)  or (iv), the Company shall promptly mail to
each Holder of  Debentures in  accordance  with Section  5(h), a notice  setting
forth the Initial  Conversion  Price after such  adjustment  and setting forth a
brief statement of the facts requiring such adjustment.


                                       10







                      (viii) In case of any reclassification of the Common Stock
or any compulsory share exchange pursuant to which the Common Stock is converted
into other securities, cash or property, the Holder of this Debenture shall have
the right  thereafter  to,  at its  option,  (A)  convert  the then  outstanding
principal  amount,  together with all accrued but unpaid  interest and any other
amounts then owing  hereunder in respect of this  Debenture only into the shares
of stock and other securities, cash and property receivable upon or deemed to be
held by holders of the Common Stock  following  such  reclassification  or share
exchange, and the Holders of the Debentures shall be entitled upon such event to
receive such amount of securities,  cash or property as the shares of the Common
Stock of the Company into which the then outstanding principal amount,  together
with all accrued but unpaid  interest and any other amounts then owing hereunder
in respect of this Debenture could have been converted immediately prior to such
reclassification  or share  exchange would have been entitled or (B) require the
Company to prepay,  from funds  legally  available  therefor at the time of such
prepayment,  all of its  Debentures  at a  price  equal  to the  sum of (i)  the
aggregate  of the  principal  amount  of  Debentures  then  held  by the  Holder
multiplied  by (a) the average Per Share  Market  Value for the five (5) Trading
Days  immediately  preceding (1) the effective date of the  reclassification  or
share exchange  triggering such  prepayment  right or (2) the date of payment in
full by the Company of the  prepayment  price  hereunder,  whichever is greater,
divided by (b) the Conversion Price calculated on such effective date, plus (ii)
the aggregate of all accrued but unpaid  interest and other amounts then payable
in respect of all Debentures to be repaid hereunder. The entire redemption price
shall be paid in cash, and the terms of payment of such  redemption  price shall
be subject to the  provisions  set forth in Section 5(b).  The terms of any such
reclassification or share exchange shall include such terms so as to continue to
give to the Holder the right to receive the  securities,  cash or  property  set
forth in this Section  4(c)(vii) upon any conversion  following such event. This
provision  shall  similarly  apply  to  successive  reclassifications  or  share
exchanges.

                           (ix)     If:

                                    A.     the Company  shall declare a dividend
                                           (or any  other  distribution)  on its
                                           Common Stock; or

                                    B.     the Company  shall  declare a special
                                           nonrecurring  cash  dividend  on or a
                                           redemption (other than redemptions of
                                           the  stock of  employees  upon  their
                                           termination  of  employment  with the
                                           Company in an aggregate amount not to
                                           exceed   $1,000,000,   calculated  by
                                           reference  to the market price of the
                                           Common  Stock  at such  time)  of its
                                           Common Stock; or

                                    C.     the  Company   shall   authorize  the
                                           granting to all holders of the Common
                                           Stock rights or warrants to subscribe
                                           for or purchase any shares of capital
                                           stock of any class or of any  rights;
                                           or


                                       11








                                    D.     the approval of any  stockholders  of
                                           the  Company  shall  be  required  in
                                           connection with any  reclassification
                                           of the Common  Stock of the  Company,
                                           any  consolidation or merger to which
                                           the  Company is a party,  any sale or
                                           transfer of all or substantially  all
                                           of the assets of the Company,  of any
                                           compulsory  share of exchange whereby
                                           the Common  Stock is  converted  into
                                           other  securities,  cash or property;
                                           or

                                    E.     the  Company   shall   authorize  the
                                           voluntary or involuntary dissolution,
                                           liquidation  or  winding  up  of  the
                                           affairs of the Company;

then the Company shall cause to be filed at each office or agency maintained for
the purpose of conversion of the Debentures, and shall cause to be mailed to the
Holders of  Debentures  at their last  addresses  as they shall  appear upon the
stock books of the Company,  at least 30 calendar  days prior to the  applicable
record or effective date hereinafter specified, a notice stating (x) the date on
which a record is to be taken for the  purpose of such  dividend,  distribution,
redemption,  rights or warrants,  or if a record is not to be taken, the date as
of which the  holders  of the  Common  Stock of record  to be  entitled  to such
dividend, distributions,  redemption, rights or warrants are to be determined or
(y) the  date on  which  such  reclassification,  consolidation,  merger,  sale,
transfer or share  exchange is expected to become  effective  or close,  and the
date as of which it is expected that holders of the Common Stock of record shall
be entitled to exchange their shares of the Common Stock for securities, cash or
other property deliverable upon such  reclassification,  consolidation,  merger,
sale, transfer or share exchange;  provided,  however,  that the failure to mail
such notice or any defect therein or in the mailing thereof shall not affect the
validity of the  corporate  action  required  to be  specified  in such  notice.
Holders are entitled to convert  Debentures  during the 30-day period commencing
the date of such  notice  to the  effective  date of the event  triggering  such
notice.

                  (d) If at any time conditions  shall arise by reason of action
taken by the  Company  which in the  opinion of the Board of  Directors  are not
adequately covered by the other provisions hereof and which might materially and
adversely affect the rights of the Holders (different than or distinguished from
the effect generally on rights of holders of any class of the Company's  capital
stock) or if at any time any such  conditions are expected to arise by reason of
any action contemplated by the Company,  the Company shall mail a written notice
briefly  describing the action  contemplated and the material adverse effects of
such action on the rights of the Holders at least 30 calendar  days prior to the
effective  date of such action,  and an  Appraiser  mutually  acceptable  to the
Holders of majority in interest of the Debentures and the Company shall give its
opinion  as to the  adjustment,  if any (not  inconsistent  with  the  standards
established  in  this  Section  4),  of  the  Conversion  Price  (including,  if
necessary,  any  adjustment  as to the  securities  into  which  Debentures  may
thereafter be convertible) and any distribution which is or would be required to
preserve  without diluting the rights of the Holders.  The  determination of the
Appraiser shall be final.


                                       12







                  (e) The Company  covenants  that it will at all times  reserve
and keep available out of its authorized and unissued shares of the Common Stock
solely for the purpose of issuance upon conversion of the Debentures and payment
of interest on the  Debentures,  each as herein  provided,  free from preemptive
rights or any other actual contingent  purchase rights of persons other than the
Holders, the sum of 3,000,000 shares of Common Stock for issuance (on a pro rata
basis as described in Section  4(a)(iii) upon  conversion of Debentures  (taking
into account the  adjustments  and  restrictions of Section 4(c)) and payment of
interest hereunder in Common Stock. The Company covenants that all shares of the
Common Stock that shall be so issuable  shall,  upon issue,  be duly and validly
authorized,  issued and fully paid,  nonassessable and, if the Underlying Shares
Registration  Statement has been declared  effective  under the Securities  Act,
freely tradeable.

                  (f) Upon a  conversion  hereunder  the  Company  shall  not be
required to issue stock  certificates  representing  fractions  of shares of the
Common Stock, but may if otherwise permitted,  make a cash payment in respect of
any final  fraction of a share based on the Per Share Market Value at such time.
If the Company elects not, or is unable, to make such a cash payment, the holder
shall be  entitled  to receive,  in lieu of the final  fraction of a share,  one
whole share of Common Stock.

                  (g) The  issuance  of  certificates  for  shares of the Common
Stock on  conversion  of the  Debentures  shall be made  without  charge  to the
Holders thereof for any  documentary  stamp or similar taxes that may be payable
in respect  of the issue or  delivery  of such  certificate,  provided  that the
Company  shall not be  required to pay any tax that may be payable in respect of
any transfer  involved in the issuance and delivery of any such certificate upon
conversion  in a name  other  than  that of the  Holder  of such  Debentures  so
converted  and the  Company  shall  not be  required  to issue or  deliver  such
certificates  unless or until the  person or  persons  requesting  the  issuance
thereof  shall  have paid to the  Company  the  amount of such tax or shall have
established to the satisfaction of the Company that such tax has been paid.

                  (h) Any and all notices or other  communications or deliveries
to be provided by the Holders of the Debentures  hereunder,  including,  without
limitation, any Conversion Notice, shall be in writing and delivered personally,
by facsimile,  sent by a nationally recognized overnight courier service or sent
by certified or registered mail, postage prepaid,  addressed to the Company,  at
10 Forge Park,  Franklin,  MA 02038 (facsimile number (508) 581-7990,  attention
Chief Executive Officer with copy to Mintz,  Levin,  Cohn,  Ferris,  Glovsky and
Popeo,  P.C.,  attention Neil H. Aronson  (facsimile number (617) 542-2241),  or
such other  address or  facsimile  number as the  Company  may  specify for such
purposes by notice to the Holders delivered in accordance with this Section. Any
and all  notices or other  communications  or  deliveries  to be provided by the
Company  hereunder shall be in writing and delivered  personally,  by facsimile,
sent by a nationally  recognized  overnight courier service or sent by certified
or registered mail, postage prepaid,  addressed to each Holder of the Debentures
at the  facsimile  telephone  number or address of such Holder  appearing on the
books of the  Company,  or if no such  facsimile  telephone  number  or  address
appears,  at the 


                                       13








principal place of business of the holder. Any notice or other  communication or
deliveries  hereunder shall be deemed given and effective on the earliest of (i)
the date of  transmission,  if such notice or  communication  is  delivered  via
facsimile at the facsimile  telephone  number specified in this Section prior to
4:30 p.m. (New York City time), (ii) the date after the date of transmission, if
such  notice or  communication  is  delivered  via  facsimile  at the  facsimile
telephone  number  specified in this Section later than 4:30 p.m. (New York City
time) on any date and earlier than 11:59 p.m. (New York City time) on such date,
(iii) four days after deposit in the United States mails,  (iv) the Business Day
following  the  date of  mailing,  if send by  nationally  recognized  overnight
courier service,  or (v) upon actual receipt by the party to whom such notice is
required to be given.

                  Section 5.   Company Conversion.

                  Provided  that the  average  Per  Share  Market  Value for any
consecutive 30 Trading Day period  commencing on the Original Issue Date exceeds
the  Initial  Conversion  Price  by more  than  50%,  this  Debenture  shall  be
convertible  in whole or in part into a number of shares of Common  Stock at the
Conversion  Price at the  option of the  Company;  provided,  however,  that the
Company is not  permitted  to deliver a Company  Conversion  Notice (as  defined
below) within 10 days after issuing any press release or other public  statement
relating  to such  conversion  or at any  time  when the  Underlying  Securities
Registration  Statement is not then  effective or shares of Common Stock are not
listed for trading.  The Company  shall effect such  conversion by delivering to
the  Holder a written  notice  in the form  attached  hereto  as  Exhibit B (the
"Company Conversion Notice"), which Company Conversion Notice, once given, shall
be  irrevocable.  Each Company  Conversion  Notice shall  specify the  principal
amount together with accrued interest of Debentures to be converted. The Company
shall  deliver  such  Company  Conversion  Notice at least two (2) Trading  Days
before the Maturity  Date or the date of  conversion  (such date is  hereinafter
referred to as the  "Company  Conversion  Date").  Upon its receipt of a Company
Conversion Notice, the Holder shall surrender the principal amount of Debentures
subject to such notice at the office of the Company or of any transfer agent for
the  Debentures  or Common  Stock.  If the Company is  converting  less than the
aggregate principal amount of all Debentures, the Company shall, upon conversion
of the principal amount of Debentures  subject to such Company Conversion Notice
and receipt of the Debentures surrendered for conversion, deliver to the Holder,
a replacement Debenture for such principal amount of Debentures as have not been
converted. Each of a Holder Conversion Notice and a Company Conversion Notice is
sometimes  referred to herein as a  "Conversion  Notice,"  and each of a "Holder
Conversion Date" and a "Company Conversion Date" is sometimes referred to herein
as a "Conversion Date."

                  Section 6. Definitions. For the purposes hereof, the following
terms shall have the following meanings:

                  "Adjustment  Amount" is equal to (i) the  aggregate  principal
amount of Debentures  then  outstanding  multiplied by (A) the average Per Share
Market Value for the five Trading Days immediately  preceding (1) the applicable
Trigger  Date or (2) the date of payment of all  amounts due as a result of such
Event of Default, whichever is greater, divided 

  
                                       14







by (B) the Conversion  Price with respect to the aggregate  principal  amount of
Debentures then  outstanding  calculated on the applicable  Trigger Date,  minus
(ii) the aggregate  principal  amount of Debentures then  outstanding,  plus all
accrued and unpaid interest  thereon and all other amounts due, except for those
referred to in (i) above pursuant to the terms hereof.

                  "Business Day" means any day except  Saturday,  Sunday and any
day which shall be a legal holiday or a day on which banking institutions in New
York City or in the  Province  of British  Columbia,  Canada are  authorized  or
required by law or other government action to close.

                  "Common Stock" means the Company's  common stock,  without par
value,  of the  Company  and stock of any other class into which such shares may
hereafter have been reclassified or changed.

                  "Original  Issue  Date"  shall  mean  the  date  of the  first
issuance  of  any  Debentures  regardless  of the  number  of  transfers  of any
Debenture  and  regardless of the number of  instruments  which may be issued to
evidence such Debenture.

                  "Per Share Market Value" means on any particular  date (a) the
closing  bid price per share of the  Common  Stock on such date on the  American
Stock Exchange or other stock  exchange or quotation  system on which the Common
Stock is then listed or if there is no such price on such date, then the closing
bid price on such  exchange or quotation  system on the date  nearest  preceding
such date,  or (b) if the Common Stock is not listed then on the American  Stock
Exchange or any stock exchange or quotation system,  the closing bid price for a
share of Common Stock in the over-the-counter  market, as reported by the Nasdaq
Stock  Market  or in the  National  Quotation  Bureau  Incorporated  or  similar
organization or agency  succeeding to its functions of reporting  prices) at the
close of business on such date,  or (c) if the Common Stock is not then reported
by the National Quotation Bureau Incorporated (or similar organization or agency
succeeding to its functions of reporting prices),  then the average of the "Pink
Sheet" quotes for the relevant conversion period, as determined in good faith by
the  Holder,  or (d) if the Common  Stock is not then  publicly  traded the fair
market value of a share of Common Stock as determined  by an appraiser  selected
in good faith by the  Holders  of a  majority  in  interest  of the  Debentures;
provided,  however, that the Company, after receipt of the determination by such
appraiser,  shall  have the right to select an  additional  appraiser,  in which
case, the fair market value shall be equal to the average of the  determinations
by each such appraiser.

                  "Person" means a corporation,  an association,  a partnership,
organization,  a business, an individual,  a government or political subdivision
thereof or a governmental agency.

                  "Registration  Rights Agreement" means the Registration Rights
Agreement,  dated July 17, 1997,  among the Company and the original  holders of
Debentures.


                                       15







                  "Trading  Day"  means (a) a day on which the  Common  Stock is
traded on the American Stock Exchange or other stock exchange or market on which
the Common  Stock has been  listed,  or (b) if the Common Stock is not listed on
the Nasdaq SmallCap  Market or any stock exchange or market,  a day on which the
Common Stock is traded in the  over-the-counter  market,  as reported by the OTC
Bulletin  Board,  or (c) if the Common  Stock is not quoted on the OTC  Bulletin
Board, a day on which the Common Stock is quoted in the over-the-counter  market
as  reported  by the  National  Quotation  Bureau  Incorporated  (or any similar
organization or agency succeeding its functions of reporting prices).

                  "Trigger  Date"  shall mean,  (i) with  respect to an Event of
Default  caused by an event  described in Section 3(a),  the date the payment of
principal or interest at issue was due, (ii) with respect to an Event of Default
caused by an event  described in Section 3(b),  the date  specified in any other
provision of this Debenture,  the Purchase Agreement or the Registration  Rights
Agreement that require  prepayment of the outstanding  principal  amount of this
Debenture as a result of an event so  contemplated,  if not, the date such event
becomes an Event of Default  pursuant to Section 3(b), and (iii) with respect to
an Event of Default caused by an event  described in Section 3(c),  (d), (e) and
(f), the date such event becomes an Event of Default pursuant to such Sections.

                  "Underlying Shares" means the number of shares of Common Stock
into which the  Debentures are  convertible in accordance  with the terms hereof
and the Purchase Agreement.

                  Section 7. Except as expressly  provided herein,  no provision
of this Debenture shall alter or impair the obligation of the Company,  which is
absolute  and  unconditional,  to pay the  principal  of, and  interest on, this
Debenture at the time,  place,  and rate,  and in the coin or  currency,  herein
prescribed. This Debenture is a direct obligation of the Company. This Debenture
ranks pari passu with all other  Debentures  now or  hereafter  issued under the
terms set forth herein.  The Company may only voluntarily prepay the outstanding
principal amount on the Debentures in accordance with Section 5 hereof.

                  Section 8. This Debenture  shall not entitle the Holder to any
of the rights of a stockholder of the Company, including without limitation, the
right to vote, to receive dividends and other  distributions,  or to receive any
notice of, or to attend,  meetings of stockholders  or any other  proceedings of
the Company,  unless and to the extent  converted into shares of Common Stock in
accordance with the terms hereof.

                  Section 9. If this Debenture shall be mutilated,  lost, stolen
or  destroyed,   the  Company  shall  execute  and  deliver,   in  exchange  and
substitution for and upon cancellation of a mutilated  Debenture,  or in lieu of
or in substitution for a lost,  stolen or destroyed  debenture,  a new Debenture
for the  principal  amount  of this  Debenture  so  mutilated,  lost,  stolen or
destroyed but only upon receipt of evidence of such loss,  theft or  destruction
of such Debenture, and of the ownership hereof, and indemnity, if requested, all
reasonably satisfactory to the Company.


                                       16







                  Section 10. This Debenture  shall be governed by and construed
in accordance  with the laws of the State of New York,  without giving effect to
conflicts of laws thereof.

                  Section  11.  Any  waiver by the  Company  or the  Holder of a
breach of any provision of this  Debenture  shall not operate as or be construed
to be a waiver of any other  breach of such  provision  or of any  breach of any
other provision of this  Debenture.  The failure of the Company or the Holder to
insist  upon  strict  adherence  to any  term of this  Debenture  on one or more
occasions  shall not be  considered  a waiver or deprive that party of the right
thereafter  to insist  upon strict  adherence  to that term or any other term of
this Debenture. Any waiver must be in writing.

                  Section 12. If any  provision  of this  Debenture  is invalid,
illegal or unenforceable,  the balance of this Debenture shall remain in effect,
and if any provision is  inapplicable  to any person or  circumstance,  it shall
nevertheless remain applicable to all other persons and circumstances.

                  Section 13. Whenever any payment or other obligation hereunder
shall be due on a day other than a Business  Day,  such payment shall be made on
the next succeeding Business Day (or, if such next succeeding Business Day falls
in the next  calendar  month,  the  preceding  Business  Day in the  appropriate
calendar month).

                   [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK
                            SIGNATURE PAGE TO FOLLOW]


                                       17







                  IN WITNESS WHEREOF,  the Company has caused this instrument to
be duly executed by an officer  thereunto  duly  authorized as of the date first
above indicated.


                                        PLC SYSTEMS INC.



                                        By:________________________________
                                             Name:
                                             Title:

Attest:



By:___________________________
   Name:
   Title:

                                       18







EXHIBIT A

                              NOTICE OF CONVERSION

(To be Executed by the Registered Holder
in order to Convert the Debenture)

The undersigned hereby elects to convert Debenture No. [ ] into shares of Common
Stock,  no par value per share (the "Common  Stock"),  of PLC Systems Inc.  (the
"Company")  according to the conditions hereof, as of the date written below. If
shares  are to be issued in the name of a person  other  than  undersigned,  the
undersigned  will pay all transfer  taxes  payable  with respect  thereto and is
delivering  herewith such  certificates and opinions as reasonably  requested by
the Company in  accordance  therewith.  No fee will be charged to the holder for
any conversion, except for such transfer taxes, if any.

Conversion calculations:
                            ----------------------------------------------------
                            Date to Effect Conversion


                            ----------------------------------------------------
                            Principal Amount of Debentures to be Converted


                            ----------------------------------------------------
                            Number of shares of Common Stock to be Issued


                            ----------------------------------------------------
                            Applicable Conversion Price


                            ----------------------------------------------------
                            Signature


                            ----------------------------------------------------
                            Name

                            ----------------------------------------------------
                            Address

The Company  undertakes to promptly upon its receipt of this  conversion  notice
(and, in any case prior to the time it effects the conversion requested hereby),
notify  the  converting  holder by  facsimile  of the number of shares of Common
Stock  outstanding  on such date and the number of shares of Common  Stock which
would be issuable to the holder if the conversion  requested in this  conversion
notice were effected in full,  whereupon,  if the Company  determines  that such
conversion  would  result in it  owning  in excess of 4.999% of the  outstanding
shares of Common Stock on such date,  the Company  shall convert up to an amount
equal to 4.999%  of the  outstanding  shares  of  Common  Stock and issue to the
holder  one or more  certificates  representing  Debentures  which have not been
converted as a result of this provision.


                                       19








                                    EXHIBIT B

                                PLC SYSTEMS INC.

                              NOTICE OF CONVERSION
                         AT THE ELECTION OF THE COMPANY


The  undersigned  in the name and on behalf of PLC SYSTEMS INC. (the  "Company")
hereby notifies the addressee  hereof that the Company hereby elects to exercise
its right to convert the above Debenture No. [ ] into shares of Common Stock, no
par  value  per share  (the  Common  Stock ), of the  Company  according  to the
conditions  hereof,  as of the date written below. No fee will be charged to the
Holder for any conversion  hereunder,  except for such transfer  taxes,  if any,
which may be incurred by the Company if shares are to be issued in the name of a
person other than the person to whom this notice is addressed.

Conversion calculations:
                            ----------------------------------------------------
                            Date to Effect Conversion


                            ----------------------------------------------------
                            Principal Amount of Debentures to be Converted


                            ----------------------------------------------------
                            Number of shares of Common Stock to be Issued


                            ----------------------------------------------------
                            Applicable Conversion Price


                            ----------------------------------------------------
                            Signature


                            ----------------------------------------------------
                            Name:


                            ----------------------------------------------------
                            Address: