EXHIBIT 10.6
 
     The securities represented hereby have not been registered under the 
Securities Act of 1933, as amended, or registered or qualified under any state
securities laws, and such securities may not be sold, transferred or otherwise
disposed of in the absence of an effective registration statement under such Act
and registration and qualification under all applicable state securities laws or
pursuant to exemptions therefrom.



                           Photoelectron Corporation
                             400-1 Totten Pond Road
                          Waltham, Massachusetts 02154



                            STOCK PURCHASE WARRANT


Date of Issuance: December 28, 1991     Right to Purchase 214,667 
                                        Shares of Common Stock 
                                         (subject to adjustment)

Warrant #1

     For value received, Photoelectron Corporation, a Massachusetts corporation
(the "Company"), hereby grants to Photoelectron Investments Corporation of
Liberia, or its registered assigns (the "Registered Holder"), the right to
purchase from the Company 214,667 shares of the Company's Common Stock (subject
to adjustment pursuant to Section 4 hereof) at a price of $1.50 per share (as
adjusted pursuant to Section 3 hereof, the "Exercise Price").  The amount and
kind of securities purchasable pursuant to the rights granted under this Warrant
and the purchase price for such securities are subject to adjustment pursuant to
the provisions contained in this Warrant.

     This Warrant is subject to the following provisions:

     1.  Definitions.  As used in this Warrant, the following terms have the
         ----------- 
meanings set forth below:

         "Common Stock" means the Company's Common Stock, $.0l par value per
          ------------ 
share.


 
                                      -2-


            "Date of Issuance" shall have the meaning specified in Section 10 of
            ------------------ 
this Warrant.

          "Market Price" is defined as the average of the daily closing prices
          --------------
for the 20 consecutive trading days, immediately preceding the date of
computation.  The closing price for each day shall be (i) if the shares of
Common Stock are listed or admitted to trading on a principal national
securities exchange or the National Market System of NASDAQ, the last reported
sales price on the principal national securities exchange on which the shares of
Common Stock are listed or admitted to trading or on the National Market System
of NASDAQ or (ii) if the shares of Common Stock are not listed or admitted to
trading on any such exchange, the average of the highest bid and lower asked
prices, as reported on the Automated Quotation System of the National Quotations
Bureau, Incorporated or an equivalent, generally accepted reporting service.  If
at any time such security is not listed on any domestic securities exchange or
quoted in the NASDAQ System or the domestic over-the-counter market, the "Market
Price" will be the fair value thereof determined by the Board of Directors in
good faith.

          "NASDAQ System" means the NASDAQ Inter-Dealer Quotation System or such
          ---------------
other similar inter-dealer quotation system as may in the future be used
generally by members of the National Association of Securities Dealers, Inc.,
for the over-the-counter transactions in securities.

          "Person" means an individual, a partnership, a corporation, a trust, a
          -------- 
joint venture, an unincorporated organization and a government or any department
or agency thereof.

          "Warrant" or "Warrants" means this Warrant and all stock purchase
          ---------    ----------
warrants issued in exchange therefor pursuant to the terms thereof.

          "Warrant Stock" means shares of the Company's authorized but unissued
          ---------------
Common Stock; provided that if there is a change such that the securities
              --------
issuable upon exercise of the Warrant are issued by an entity other than the
Company or there is a change in the class of securities so issuable, then the
term "Warrant Stock" will mean one share of the security issuable upon exercise
of the Warrant if such security is issuable in shares, or will mean the smallest
unit in which such security is issuable if such security is not issuable in
shares.

 
                                      -3-

     2.  Exercise of Warrant.
         --------------------

         2.1  Exercise Period.  The Registered Holder may exercise this
              ---------------
Warrant, in whole or in part (but not as to a fractional share of Warrant
Stock), at any time and from time to time prior to the fifth anniversary of the
Date of Issuance of the Warrant (the "Exercise Period").

         2.2  Exercise Procedure.
              ------------------

              (a)  This Warrant will be deemed to have been exercised at such
time as the Company has received all of the following items (the "Exercise 
Date"):

                   (i)    a completed Exercise Agreement, as described below,
              executed by the Person exercising all or part of the purchase
              right represented by this Warrant (the "Purchaser");

                   (ii)   this Warrant;

                   (iii)  if this Warrant is not registered in the name of the
              Purchaser, an Assignment or Assignments in the form set forth in
              Exhibit II hereto, evidencing the assignment of this Warrant to
              the Purchaser; and

                   (iv)   a check payable to the Company in an amount equal
              to the product of the Exercise Price multiplied by the number of
              shares of Warrant Stock being purchased upon such exercise.

              (b)  Certificates for shares of Warrant Stock purchased upon 
exercise of this Warrant will be delivered by the Company to the Purchaser
within ten days after the Exercise Date. Unless this Warrant has expired or all
of the purchase rights represented hereby have been exercised, the Company will
prepare a new Warrant, substantially identical hereto, representing the rights
formerly represented by this Warrant which have not expired or been exercised.
The Company will, within such ten-day period, deliver such new Warrant to the
Person designated for delivery in the Exercise Agreement.

              (c)  The Warrant Stock issuable upon the exercise of this Warrant
will be deemed to have been issued to the Purchaser on the Exercise Date, and
the Purchaser will be deemed for all purposes to have been the record holder of
such Warrant Stock on the Exercise Date.

              (d)  The issuance of certificates for shares of Warrant Stock upon
exercise of this Warrant will be made without charge to the Registered Holder or
the Purchaser for

 
                                      -4-

any issuance tax in respect thereof or any other cost incurred by the Company in
connection with such exercise and the related issuance of shares of Warrant
Stock.  The Company shall not, however, be required to pay any tax which may be
payable in respect of any transfer, in whole or in part, of this Warrant
(including the issuance of new Warrants in connection therewith or the delivery
of stock certificates in a name other than that of the Registered Holder of this
Warrant presented for exercise, and any such tax shall be paid by such 
Registered Holder at the time of presentation.

              (e)  The Company will not close its books for the transfer of this
Warrant or of any share of Warrant Stock issued or issuable upon the exercise on
this Warrant in any manner which interferes with the timely exercise of this
Warrant.

        2.3   Exercise Agreement.  The Exercise Agreement will be substantially
              ------------------
in the form set forth in Exhibit I hereto, except that if the shares of Warrant
Stock are not to be issued in the name of the Registered Holder of this
Warrant, the Exercise Agreement will also state the name of the Person to whom
the certificates for the shares of Warrant Stock are to be issued, and if the
number of shares of Warrant Stock to be issued does not include all the shares
of Warrant Stock purchasable hereunder, it will also state the name of the
Person to whom a new Warrant for the unexercised portion of the rights hereunder
is to be delivered.

        2.4   Fractional Shares.  If a fractional share of Warrant Stock
              -----------------
would, but for the provisions of Subsection 2.1, be issuable upon exercise of
the rights represented by this Warrant, the Company will, within ten days after
the Exercise Date, delivery to the Purchaser a check payable to the Purchaser in
lieu of such fractional share, in an amount equal to the Market Price of such
fractional share as of the close of business on the Exercise Date.

     3. Exercise Price.
        --------------

        3.1   General.  The initial Exercise Price will be $1.50.  In order to
              -------
prevent dilution of the rights granted under this Warrant, the Exercise Price
will be subject to adjustment from time to time pursuant to this Section 3.

        3.2   Subdivision or Combination of Common Stock and Stock Dividends.
              --------------------------------------------------------------
In case the Company shall at any time after the date hereof (a) issue any shares
of Common Stock as a dividend upon Common Stock, or (b) issue any shares of
Common Stock by reclassification or otherwise, or (c) combine outstanding shares
of Common Stock, by reclassification or otherwise, the Exercise Price which
would apply if purchase rights hereunder were being exercised immediately prior
to such

 
                                      -5-

action by the Company shall be adjusted by multiplying it by a fraction, the
numerator of which shall be the number of shares of Common Stock outstanding
immediately prior to such dividend, subdivision or combination and the
denominator of which shall be the number of shares of Common Stock outstanding
immediately after such dividend, subdivision or combination.

          3.3  Certain Dividends.  In case the Company shall declare a dividend
               -----------------
upon the Common Stock payable otherwise than out of earnings or retained
earnings and otherwise than in Common Stock, the Exercise Price shall be
adjusted by multiplying the Exercise Price in effect immediately prior to the
declaration of such dividend by a fraction, the numerator of which shall be the
current Market Price per share of Common Stock, on such date, less the fair
market value, as determined by the Board of Directors of the Company, whose
determination shall be conclusive, of the portion of the assets or evidences of
indebtedness so to be distributed or of such subscription rights, options or
warrants applicable to one share of Common Stock, and of which the denominator
shall be such current Market Price per share of Common Stock. For the purposes
of the foregoing, a dividend other than in cash shall be considered payable out
of earnings or retained earnings only to the extent that such earnings or
retained earnings are charged an amount equal to the value of such dividend as
determined by the Board of Directors of the Company.  Such reductions shall take
effect as of the date on which a record is taken for the purpose of such
dividend, or, if a record is not taken, the date as of which the holders of
Common Stock or record entitled to such dividend are to be determined.

        3.4   No Adjustments.  No adjustment of the Exercise Price shall be
              --------------
made if the amount of such adjustment shall be less than one cent per share, but
in such case any adjustment that would otherwise be required then to be made
shall be carried forward and shall be made at the time and together with the
next subsequent adjustment which, together with any adjustment or adjustments so
carried forward, shall amount to not less than one cent per share.

    4.  Adjustment of Number of Shares Issuable upon Exercise.  In the event of
        -----------------------------------------------------
a stock dividend, stock split, combination or other event described in Sections
3.2 and 3.3 hereof, the Registered Holder of this Warrant shall thereafter
(until another such adjustment) be entitled to purchase the number of shares of
Warrant Stock, calculated to the nearest full share, determined by (a)
multiplying the number of shares of Warrant Stock purchasable hereunder
immediately prior to the adjustment of the Exercise Price by the Exercise Price
in effect immediately prior to such adjustment, and (b) dividing the product so
obtained by the adjusted Exercise Price in effect immediately after such
adjustment.

 
                                      -6-

    5.  Effect of Reorganization, Reclassification, Consolidation, Merger or
        --------------------------------------------------------------------
Sale.  If at any time while this Warrant is outstanding there shall be any
- ----
reorganization or reclassification of the capital stock of the Company (other
than a subdivision or combination of shares provided for in Subsection 3.3
hereof) or any consolidation or merger of the Company with another corporation
(other than a consolidation or merger in which the Company is the surviving
entity  and which does not result in any change in the Common Stock),  or any
sale or other disposition by the Company of all or substantially all of its
assets to any other corporation, the holder of this Warrant shall thereafter
upon exercise of this Warrant be entitled to receive the number of shares of
stock or other securities or property of the Company, or of the successor
corporation resulting from such consolidation or merger, as the case may be, to
which the Warrant Common Stock (and any other securities and property) of the
Company, deliverable upon the exercise of this Warrant, would have been entitled
upon such reorganization, reclassification of capital stock, consolidation,
merger, sale or other disposition if this Warrant had been exercised immediately
prior to such reorganization, reclassification of capital stock, consolidation,
merger, sale or other disposition.  In any such case, appropriate adjustment (as
determined by the Board of Directors of the Company) shall be made in the
application of the provisions set forth in this Warrant with respect to the
rights and interests thereafter of the holder of this Warrant to the end that
the provisions set forth in this Warrant (including those relating to
adjustments of the Exercise Price and the number of shares issuable upon the
exercise of this Warrant) shall thereafter be applicable, as near as reasonably
may be, in relation to any shares or other property thereafter deliverable upon
the exercise hereof as if this Warrant had been exercised immediately prior to
such reorganization, reclassification of capital stock, consolidation, merger,
sale or other disposition and the holder hereof had carried out the terms of the
exchange as provided for by such reorganization, reclassification of capital
stock, consolidation or merger. Notwithstanding any other provisions of this
Warrant, in the event of sale or other disposition of all or substantially all
of the assets of the Company as a part of a plan for liquidation of the Company,
all rights to exercise the Warrant shall terminate 30 days after the Company
gives written notice to the Registered Holder of this Warrant that such sale or
other disposition has been consummated.

    6.  Notice of Adjustments.  Immediately upon any adjustment of the Exercise
        ---------------------
Price or increase or decrease in the number of shares of Common Stock
purchasable upon exercise of this Warrant, the Company will send written notice
thereof to all Registered Holders, stating the adjusted Exercise Price and the
increased or decreased number of shares purchasable upon

 
                                      -7-

exercise of this Warrant and setting forth in reasonable detail the method of
calculation for such adjustment and increase or decrease.

    7.  Reservation of Common Stock.  The Company will at all times reserve and
        ---------------------------
keep available for issuance upon the exercise of Warrants such number of its
authorized but unissued shares of Common Stock as will be sufficient to permit
the exercise in full of all outstanding Warrants, and upon such issuance such
shares of Common Stock will be validly issued, fully paid and nonassessable.

    8.  No Voting Rights; Limitations of Liability.  This Warrant will not
        ------------------------------------------
entitle the holder hereof to any voting rights or other rights as a stockholder
of the Company.  No provision of this Warrant, in the absence of affirmative
action by the Registered Holder to purchase Warrant Stock, and no enumeration in
this Warrant of the rights or privileges of the Registered Holder, will give
rise to any liability of such Holder for the Exercise Price of Warrant Stock
acquirable by exercise hereof or as a stockholder of the Company.

    9.  Warrant Transferable.
        --------------------

        (a)   Subject to the transfer conditions referred to in paragraph (b),
below, this Warrant and all rights hereunder are transferable, in whole or in
part, without charge to the Registered Holder, upon surrender of this Warrant
with a properly executed Assignment (in the form of Exhibit II hereto) at the
principal office of the Company.

        (b)   Each Registered Holder of this Warrant acknowledges that this
Warrant has not been registered under the Securities Act of 1933, as amended
(the "Act"), and agrees not to sell, pledge, distribute, offer for sale,
transfer or otherwise dispose of this Warrant or any Warrant Stock issued upon
its exercise in the absence of (i) an effective registration statement as to
this Warrant or such Warrant Stock under the Act (or any similar statute then in
effect), or (ii) an opinion of counsel for the Company to the effect that such
registration is not, under the circumstances, required.

    10. Warrant Exchangeable for Different Denominations. This Warrant is
        ------------------------------------------------
exchangeable, upon the surrender hereof by the Registered Holder at the
principal office of the Company, for new Warrants of like tenor representing in
the aggregate the purchase rights hereunder, and each of such new Warrants will
represent such portion of such rights as is designated by the Registered Holder
at the time of such surrender.  The date the Company initially issues this
Warrant will be deemed to be the "Date of Issuance" of this Warrant regardless
of the number of times new certificates representing the unexpired and
unexercised rights formerly represented by this Warrant are issued.

 
                                      -8-

    11.  Representations, Warranties and Covenants of the Registered Holder.
         ------------------------------------------------------------------
(a)  The Registered Holder represents and warrants to, and covenants with, the
Company that:  (i)  the Registered Holder, taking into account the personnel and
resources it can practically bring to bear on the purchase of the Warrant, is
knowledgeable, sophisticated and experienced in making, and is qualified to
make, decisions with respect to investments in shares presenting an investment
decision like that involved in the purchase of the Warrant, including
investments in securities issued by the Company, and has requested, received,
reviewed and considered all information it deems relevant in making an informed
decision to purchase the Warrant; (ii) the Registered Holder is acquiring the
Warrant for investment and with no present intention of distributing the Warrant
(this representation and warranty not limiting the Registered Holder 's right to
transfer all or any part of the Warrant pursuant to Section 9); and (iii) the
Registered Holder will not, directly or indirectly, voluntarily offer, sell,
pledge, purchase or otherwise dispose of (or solicit any offers to buy, purchase
or otherwise acquire or take a pledge of) the Warrant except in compliance with
the Securities Act, and the rules and regulations promulgated thereunder, or an
exception thereto.

    (b) The Registered Holder further represents and warrants to, and covenants
with, the Company that (i) the Registered Holder has full right, power,
authority and capacity to enter into this Warrant and to consummate the
transactions contemplated hereby, and (ii) upon the execution and delivery of
this Warrant (for the limited purpose of this Section 11), this Warrant shall
constitute a valid and binding obligation of the Registered Holder enforceable
in accordance with its terms, except as enforceability may be limited by
applicable bankruptcy, insolvency, reorganization, moratorium or similar laws
affecting creditors' and contracting parties' rights generally and except as
enforceability may be subject to general principles of equity (regardless of
whether such enforceability is considered in a proceeding in equity or at law). 

    12.  Miscellaneous.
         -------------

         12.1  Amendment and Waiver.  The provisions of the Warrants may be
               --------------------
amended and the Company may take any action herein prohibited, or omit to
perform any act herein required to be performed by it, only if the Company has
obtained the written consent of the Registered Holders of Warrants representing
at least 50% of the shares of Warrant Stock obtainable upon the exercise of the
Warrants outstanding at the time of such consent.

         12.2  Notices.  Any notices required to be sent to a Registered Holder
               -------
will be delivered to the address of such Registered Holder shown on the books of
the Company.  All

 
                                      -9-

notices referred to herein will be delivered in person or sent by first class
mail, postage prepaid, and will be deemed to have been given when so delivered
or sent.

         12.3  Descriptive Headings; Governing Law.  The descriptive headings
               -----------------------------------
of the paragraphs of this Warrant are inserted for convenience only and do not
constitute a part of this Warrant.  The construction, validity and
interpretation of this Warrant will be governed by the laws of the Commonwealth
of Massachusetts.

     IN WITNESS WHEREOF, the Company and the Registered Holder (for the limited
purpose of Section 11 hereof) have caused this Warrant to be signed and attested
by their duly authorized officers, and in the case of the Company, under its
corporate seal.


                               PHOTOELECTRON CORPORATION



                               By: /s/ Peter M. Nomikos    
                                  -------------------------------
                                  Name:
                                  Title:


[Corporate Seal]

Attest:


[SIGNATURE APPEARS HERE]
- -------------------------------
Secretary



                               Accepted and agreed to by the
                               Registered Holder for the limited
                               purpose of Section 11 hereof:

                               PHOTOELECTRON INVESTMENTS
                               CORPORATION OF LIBERIA



                               By: /s/ Peter M. Nomikos       
                                  -------------------------------
                                  Name:
                                  Title:

[131d]

 
                                      -10-

                                                                EXHIBIT I
                                                                ---------


                              EXERCISE AGREEMENT
                              ------------------


To:  Photoelectron Corporation                      Date:



     The undersigned, pursuant to the provisions set forth in the within
Warrant, hereby agrees to subscribe for and purchase ________ shares of the
Warrant Stock covered by such Warrant and makes payment herewith in full for
such Warrant Stock at the price per share provided by such Warrant.



                                       Name
                                           -------------------------------
                                           -------------------------------
                                       Signature
                                                --------------------------
                                                --------------------------
                                       Address  
                                                --------------------------
                                                --------------------------


     This Exercise Agreement must be completed and sent with the items specified
in Section 2.2 of the Warrant Agreement to:

                           Photoelectron Corporation
                            400-1 Totten Pond Road
                         Waltham, Massachusetts 02154
                     Att'n:  Sandra L. Lambert, Secretary

[131d]

 
                                      -11-

                                                        EXHIBIT II
                                                        ----------



                                   ASSIGNMENT
                                   ----------



     FOR VALUE RECEIVED, ____________________ hereby sells, assigns and
transfers all of the rights of the undersigned under the within Warrant with
respect to the number of shares of the Warrant Stock covered thereby set forth
below, unto:


Names of Assignees             Address                  No. of Shares
- ------------------             -------                  -------------





Date:                          Signature______________________________
                                        ______________________________
                               
                               Witness  ______________________________


     This Assignment must be completed and sent to:

                           Photoelectron Corporation
                            400-1 Totten Pond Road
                         Waltham, Massachusetts 02154
                      Attn:  Sandra L. Lambert, Secretary

[131d]

 
                   Schedule to Exhibit 10.6 of Substantially
                   Identical Documents Omitted From Exhibits
                   ----------------------------------------- 
 
 
 
Date of    Warrant                                 No. of          Duration of
Issuance     No.            Issued To              Shares        Exercise Period
- --------   -------          ---------              ------        ---------------
                                                      
 3/28/92     2       Photoelectron Investments       240,000     Five years
                      Corporation of Liberia                              

 6/27/92     3       Photoelectron Investments       280,000     Five years 
                      Corporation of Liberia                              

 9/26/92     4       Photoelectron Investments       280,000     Five years 
                      Corporation of Liberia                              

12/28/92     5          Peter M. Nomikos             200,000     Seven years
                                                                          

 9/29/93     6          Peter M. Nomikos           1,150,000     Seven years
                                                                          

 1/27/95     7          Peter M. Nomikos             470,000     Seven years