OFFER TO PURCHASE FOR CASH
                     ALL OUTSTANDING SHARES OF COMMON STOCK
                                       OF
                         THERMO BIOANALYSIS CORPORATION
                                       AT
                              $28.00 NET PER SHARE
                                       BY
                          BIOANALYSIS ACQUISITION INC.
                          A WHOLLY-OWNED SUBSIDIARY OF
                         THERMO INSTRUMENT SYSTEMS INC.

    THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK
                                      CITY
        TIME, ON THURSDAY, APRIL 13, 2000, UNLESS THE OFFER IS EXTENDED.

To Our Clients:

    Enclosed for your consideration is an Offer to Purchase dated March 17, 2000
(the "Offer to Purchase") and the related Letter of Transmittal (which, together
with any amendments or supplements thereto, collectively constitute the "Offer")
relating to an offer by BioAnalysis Acquisition Inc., a Delaware corporation
(the "Purchaser") and a wholly-owned subsidiary of Thermo Instrument
Systems Inc., a Delaware corporation ("Thermo Instrument"), to purchase all
outstanding shares of common stock, par value $0.01 per share (the "Shares"), of
Thermo BioAnalysis Corporation, a Delaware corporation (the "Company"), at a
purchase price of $28.00 per Share, net to the seller in cash, without interest
thereon, upon the terms and subject to the conditions set forth in the Offer.

    We are the holder of record of Shares held by us for your account. A TENDER
OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND PURSUANT TO
YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR YOUR
INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER SHARES HELD BY US FOR YOUR
ACCOUNT.

    We request instructions as to whether you wish to have us tender on your
behalf any or all of the Shares held by us for your account, pursuant to the
terms and subject to the conditions set forth in the Offer.

    Your attention is directed to the following:

    - The tender price is $28.00 per Share, net to the seller in cash, without
      interest thereon.

    - The Offer and withdrawal rights will expire at 12:00 midnight, New York
      City time, on Thursday, April 13, 2000, unless the Offer is extended.

    - The Offer is made for all of the outstanding Shares.

    - The Offer is conditioned upon, among other things described in the Offer
      to Purchase, there being validly tendered and not withdrawn prior to the
      expiration of the Offer that number of Shares which, together with Shares
      owned by Thermo Electron Corporation and its subsidiaries, including
      Thermo Instrument, constitutes at least ninety percent (90%) of the
      outstanding Shares on the Expiration Date (as defined in the section of
      the Offer to Purchase captioned "The Tender Offer--Terms Of The Offer;
      Expiration Date"). The Offer is also subject to other important terms and
      conditions contained in the Offer to Purchase.

    - Tendering stockholders will not be obligated to pay brokerage fees or
      commissions or, except as set forth in Instruction 6 of the Letter of
      Transmittal, transfer taxes on the purchase of Shares by the Purchaser
      pursuant to the Offer.

    - In all cases, payment for Shares tendered and accepted for payment
      pursuant to the Offer will be made only after timely receipt by American
      Stock Transfer & Trust Company (the "Depositary") of (i) certificates
      evidencing Shares or timely confirmation of a book-entry transfer of such
      Shares into the Depositary's account

      at the Book Entry Transfer Facility (as defined in the section of the
      Offer to Purchase captioned "The Tender Offer--Acceptance For Payment And
      Payment For Shares") pursuant to the procedures set forth in the section
      of the Offer to Purchase captioned "The Tender Offer--Procedures For
      Accepting The Offer And Tendering Shares," (ii) the Letter of Transmittal
      (or a facsimile thereof), properly completed and duly executed, with any
      required signature guarantees, or an Agent's Message (as defined in the
      section of the Offer to Purchase captioned "The Tender Offer--Acceptance
      For Payment And Payment For Shares") in connection with a book-entry
      transfer and (iii) any other documents required by the Letter of
      Transmittal.

    The Offer is being made solely by the Offer to Purchase and the related
Letter of Transmittal and is being made to all holders of Shares. The Offer is
not being made to (nor will tenders be accepted from or on behalf of) holders of
Shares in any jurisdiction in which the making of the Offer or the acceptance
thereof would not be in compliance with the laws of such jurisdiction. In any
jurisdiction where the securities, blue sky or other laws require the Offer to
be made by a licensed broker or dealer, the Offer shall be deemed to be made on
behalf of the Purchaser by J.P. Morgan Securities Inc., The Beacon Group Capital
Services, LLC or one or more registered brokers or dealers licensed under the
laws of such jurisdiction.

    If you wish to have us tender any or all of the Shares held by us for your
account, please instruct us by completing, executing and returning to us the
instruction form contained in this letter. If you authorize a tender of your
Shares, all such Shares will be tendered unless otherwise specified in such
instruction form. YOUR INSTRUCTIONS SHOULD BE FORWARDED TO US IN AMPLE TIME TO
PERMIT US TO SUBMIT A TENDER ON YOUR BEHALF PRIOR TO THE EXPIRATION OF THE
OFFER.

                                       2

                        INSTRUCTIONS WITH RESPECT TO THE
                           OFFER TO PURCHASE FOR CASH
                     ALL OUTSTANDING SHARES OF COMMON STOCK
                                       OF
                         THERMO BIOANALYSIS CORPORATION

    The undersigned acknowledge(s) receipt of your letter enclosing the Offer to
Purchase dated March 17, 2000 (the "Offer to Purchase") and the related Letter
of Transmittal (which, together with any amendments or supplements thereto,
collectively constitute the "Offer") pursuant to an offer by BioAnalysis
Acquisition Inc., a Delaware corporation and a wholly-owned subsidiary of Thermo
Instrument Systems Inc., a Delaware corporation, to purchase all of the
outstanding shares of common stock, par value $0.01 per share (the "Shares"), of
Thermo BioAnalysis Corporation, a Delaware corporation.

    This will instruct you to tender the number of Shares indicated below (or,
if no number is indicated below, all Shares) that are held by you for the
account of the undersigned, upon the terms and subject to the conditions set
forth in the Offer.

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  ----------------------------------------------
  Number of Shares to be
  Tendered(1):
  ----------------------------------------------

  Dated: , 2000

                                   SIGN HERE  --       X
                                                       Signature(s):

                                                       Please type or print name(s):

                                                       Address:

                                                       Area Code and Telephone Number:

                                                       Tax Identification or Social Security Number:


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

(1) Unless otherwise indicated, it will be assumed that all of the Shares held
    by us for your account are to be tendered.

                                       3