SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                   -------------------------------------------


                                    FORM 8-K


                                 CURRENT REPORT

                       PURSUANT TO SECTION 13 OR 15(d) OF
                       THE SECURITIES EXCHANGE ACT OF 1934



                             Date of Report (Date of
                            earliest event reported):

                                 April 13, 2000
                               ------------------


                         THERMO INSTRUMENT SYSTEMS INC.
                     -------------------------------------
               (Exact Name of Registrant as Specified in Charter)


     Delaware                      1-9786                             04-2925809
- --------------------------------------------------------------------------------
(State or Other Jurisdiction  (Commission File                  (I.R.S. Employer
     of Incorporation)            Number)                    Identification No.)



81 Wyman Street, P.O. Box 9046
Waltham, MA                                                           02454-9046
- --------------------------------------------------------------------------------
(Address of Principal Executive Offices)                              (Zip Code)


       Registrant's telephone number, including area code: (781) 622-1000









         This Form 8-K contains forward-looking statements that involve a number
of risks and uncertainties. Important factors that could cause actual results to
differ materially from those indicated by such forward-looking statements are
set forth under the heading "Forward-looking Statements" in Exhibit 13 to Thermo
Instrument Systems Inc.'s annual report on Form 10-K for the year ended January
1, 2000. These include risks and uncertainties relating to: the Registrant's
growth, acquisition strategy, product development and technological change,
competition, protection of intellectual property rights, international
operations, possible changes in governmental regulations, capital spending and
government funding policies, and the Registrant's cash management arrangement
with Thermo Electron Corporation.

Item 5.  Other Events
         ------------

         On April 14, 2000, the Registrant issued a press release, attached
hereto as Exhibit 99, regarding its previously announced tender offer for the
shares of common stock, $.01 par value per share, of its Thermo BioAnalysis
Corporation subsidiary.

Item 7.  Financial Statements, Pro Forma Financial Information and Exhibits
         ------------------------------------------------------------------

         (a)      Financial Statements of Business Acquired: not applicable

         (b)      Pro Forma Financial Information: not applicable

         (c)      Exhibits:

                  99 - Press Release dated April 14, 2000






                                    SIGNATURE


         Pursuant to the requirements of the Securities Exchange Act of 1934,
the Registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized, on this 14th day of April, 2000.



                                           THERMO INSTRUMENT SYSTEMS INC.



                                            By: /s/ Theo Melas-Kyriazi
                                               ----------------------------
                                               Theo Melas-Kyriazi
                                               Chief Financial Officer





                                                                      Exhibit 99
Investor Contact: 781-622-1111
Media Contact: 781-622-1252



                  THERMO INSTRUMENT COMPLETES CASH TENDER OFFER
                             FOR THERMO BIOANALYSIS

WALTHAM, Mass., April 14, 2000 - Thermo Instrument Systems Inc. (ASE-THI), a
Thermo Electron company, announced today that it has successfully completed the
cash tender offer of $28.00 per share for any and all outstanding shares of its
Thermo BioAnalysis Corporation subsidiary (ASE-TBA).

         The offer and withdrawal rights expired at midnight on Thursday, April
13, 2000. Based on the preliminary report of the depositary for the offer,
American Stock Transfer & Trust Company, approximately 2.3 million Thermo
BioAnalysis shares were tendered. This brings Thermo Instrument's and Thermo
Electron's combined equity ownership in Thermo BioAnalysis to approximately 99.3
percent. Thermo Instrument expects to complete the spin-in of Thermo BioAnalysis
by Wednesday, April 19, through a short-form merger. The short-form merger does
not require Thermo BioAnalysis' board or shareholder approval.

         Thermo BioAnalysis shareholders who tendered their shares in the tender
offer will receive payment for their shares shortly. Thermo BioAnalysis
shareholders who did not tender their shares will also receive $28.00 per share
in the short-form merger. Information outlining what steps these shareholders
must take to obtain payment will be mailed within a week to 10 days.

         The complete terms and conditions of the tender offer are set forth in
the offer to purchase, letter of transmittal, and other related materials, which
were filed with the Securities and Exchange Commission on March 17, 2000.

         Thermo Instrument Systems Inc. is a global technology company serving
multiple markets, including the life sciences, telecommunications, food and
beverage, chemical, and oil and gas industries, with instrumentation,
information-management software, and worldwide service for a range of
applications. Our products help scientists make the discoveries that will fight
disease and prolong life. They increase the speed and quality of communications.
And they provide knowledge about the quality of materials used in manufacturing,
improve the manufacturing process, and protect the environment. More information
is available on the Internet at http://www.thermo.com/subsid/thi1.html.