[REN CORPORATION--USA LETTERHEAD]
 
                                                              September 19, 1995
 
To Our Shareholders:
 
    We are pleased to inform you that on September 12, 1995, REN
Corporation--USA (the "Company") entered into an Agreement and Plan of Merger
(the "Merger Agreement") with Gambro AB ("Gambro"), Gambro's wholly owned
subsidiary COBE Laboratories, Inc. ("COBE") and REN Acquisition Corp.
("Purchaser"), a wholly owned subsidiary of COBE, pursuant to which Purchaser
has today commenced a tender offer to purchase all of the outstanding shares of
the Company's common stock, no par value (the "Shares"), at a cash price of
$20.00 per Share (the "Offer"). The Offer is conditioned upon the tender of at
least a majority of the Shares held by shareholders other than Gambro, COBE,
Purchaser or any of their affiliates. Subject to fulfillment of the condition
that Purchaser purchase all Shares validly tendered in the Offer and certain
other conditions, the Merger Agreement provides that the Offer will be followed
by a merger (the "Merger") in which those Shares that are not acquired in the
Offer will be converted into the right to receive $20.00 per share in cash.
 
    The Board of Directors of the Company, acting upon the unanimous
recommendation of a special committee of independent directors (the "Special
Committee"), has unanimously approved and consented to the Offer and has
unanimously determined that the Merger Agreement and the transactions
contemplated thereby, including the Offer and the Merger, are fair to and in the
best interests of the holders of the outstanding Shares. The Board of Directors
has reached these conclusions after consideration of the terms of the Merger
Agreement, the unanimous recommendation of the Special Committee, the opinion of
Alex. Brown & Sons Incorporated (the independent financial advisor to the
Special Committee) dated as of September 12, 1995, that the consideration to be
received by the shareholders of the Company other than Gambro, COBE, Purchaser
and their affiliates pursuant to the Merger Agreement was fair from a financial
point of view to such shareholders as of the date of the opinion and the other
factors described in the attached Schedule 14D-9 filed today by the Company with
the Securities and Exchange Commission.
 
    Accompanying this letter, in addition to the Schedule 14D-9, is the Offer to
Purchase of the Purchaser, dated September 19, 1995, together with related
materials including a letter of transmittal for use in tendering your shares.
These documents set forth the terms and conditions of the Offer and provide
instructions as to how to tender your shares. We urge you to read the enclosed
materials carefully in making your decision whether to tender your shares to
Purchaser.
 
                                          Very truly yours,
 
                                          Lawrence J. Centella
                                          President and Chief Executive Officer