EXHIBIT 6

                                 ENDOGEN, INC.
                                30 COMMERCE WAY
                             WOBURN, MA 01801-1059

                                                                    June 2, 1999

To Our Stockholders:

    On behalf of the Board of Directors of Endogen, Inc., a Massachusetts
corporation (the "COMPANY"), we are pleased to inform you that, on May 27, 1999,
the Company entered into an Agreement and Plan of Merger (the "MERGER
AGREEMENT") with PerBio Science AB, a Swedish corporation ("PARENT") and EWOK
Acquisition Corp., a Massachusetts corporation and a wholly-owned subsidiary of
Parent ("PURCHASER"), pursuant to which Purchaser has today commenced a cash
tender offer (the "OFFER") to purchase all of the outstanding shares (the
"SHARES") of the Company's Common Stock at $3.75 per Share. Under the terms of
the Merger Agreement, the Offer will be followed by a merger of Purchaser with
and into the Company (the "MERGER") in which any Shares not tendered will be
converted into the right to receive $3.75 per Share in cash, without interest.

    Your Board of Directors has determined that the Offer and the Merger are
fair to, and in the best interests of, the Company and its stockholders, and has
approved the Offer and the Merger. The Board of Directors recommends that the
Company's stockholders accept the Offer and tender their Shares pursuant to the
Offer.

    In arriving at this recommendation, the Board of Directors gave careful
consideration to a number of factors described in the attached Schedule 14D-9
that is being filed today with the Securities and Exchange Commission. These
factors included, among other things, the terms and conditions of the Merger
Agreement and the fairness opinion of Adams, Harkness & Hill, Inc. ("AH&H"), the
Company's financial advisor, addressed to the Board of Directors, the text of
which is more fully described in the attached Schedule 14D-9. Stockholders are
urged to read the AH&H opinion in its entirety.

    Pursuant to a Stockholder Agreement dated May 27, 1999, by and among Parent,
Purchaser and the directors and certain executive officers of the Company, these
directors and executive officers have agreed to tender and not withdraw the
Shares they own in the Offer.

    In addition to the attached Schedule 14D-9 relating to the Offer, we are
enclosing the Offer to Purchase, dated June 2, 1999, of Purchaser and Parent,
together with related materials, including a Letter of Transmittal, to be used
for tendering your Shares. These documents set forth the terms and conditions of
the Offer and the Merger and provide instructions as to how to tender your
Shares. We urge you to read the enclosed materials carefully in making your
decision with respect to tendering your Shares pursuant to the Offer.

                                          On behalf of the Board of Directors,

                                          /s/ Owen A. Dempsey

                                          Owen A. Dempsey

                                          President and Chief Executive Officer