Exhibit 4.1


                        [FRONT OF STOCK CERTIFICATE]  

                           THERMEDICS DETECTION INC.

TDX                              
COMMON STOCK PAR    Incorporated under the laws of the   COMMON STOCK
PAR VALUE $.10        Commonwealth of Massachusetts      CUSIP 88355E 10 5
                                                         SEE REVERSE SIDE 
                                                         FOR CERTAIN DEFINITIONS

This certifies that


is the owner of 


 FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, $.10 PAR VALUE, OF 
                           THERMEDICS DETECTION INC.

transferable on the books of the Corporation by the holder hereof in person or
by duly authorized attorney upon surrender of this Certificate properly
endorsed. The Certificate and the shares represented hereby are issued under and
shall be subject to the laws of the Commonwealth of Massachusetts and all the
provisions of the Articles of Organization and the By-Laws of the Corporation,
and all the amendments from time to time made thereto. This Certificate is not
valid unless countersigned and registered by the Transfer Agent and Registrar.

Witness the facsimile seal of the Corporation and the facsimile signatures
of its duly authorized officers. 



Dated:



                        [  THERMEDICS DETECTION INC.  ]
                         [       Corporate Seal      ]



Treasurer                                 President and Chief Executive Officer

 
                          [BACK OF STOCK CERTIFICATE]

                           THERMEDICS DETECTION INC.

        This Corporation will furnish without charge to each stockholder who so
requests, a copy of the designations, powers, preferences and relative,
participating, optional or other special rights of each class of stock or series
thereof, and the qualifications, limitation or restrictions of such preferences
and/or rights. Any such requests may be addressed to the Clerk of the
Corporation.

        The following abbreviations when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

TEN COM  -   as tenants in common               
                                                                            
TEN ENT  -   as tenants by the entirety                                     

JT TEN   -   as joint tenants with right of
             survivorship and not as 
             tenants in common


UNIF GIFT MIN ACT - __________ Custodian ___________
                      (Cust)                (Minor)
                    under Uniform Gifts to Minors 

                    Act __________________________
                                 (State)


    Additional abbreviations may also be used though not in the above list.

For value received, ______________________ hereby sell, assign and transfer unto


PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE

_____________________________________________________________________________

_____________________________________________________________________________
(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS INCLUDING POSTAL ZIP CODE OF
ASSIGNEE)

_____________________________________________________________________________

_________________________________________________________ Shares of the Stock 

represented by the within Certificate, and do hereby irrevocably constitute
and appoint _________________________________________________________________ 

_____________________________________________________________________________
Attorney to transfer the said Stock on the books of the within-named
Corporation with full power of substitution in the premises.


Dated _______________________________   _________________________________
                                                    Signature



                                       ___________________________________
                                       THE SIGNATURES TO THIS ASSIGNMENT 
                                       MUST CORRESPOND WITH THE CERTIFICATE 
                                       IN EVERY PARTICULAR WITHOUT 
                                       ALTERATION OR ENLARGEMENT OR ANY 
                                       CHANGE WHATSOEVER.