SUMMARYEighteen beagles were each given a kidney homotransplant. One of the recipient's own kidneys was removed at the time of transplantation, the other 21 days later. Eight of the recipients received a daily injection of horse anti-dog antilymphocytic serum (ALS), 5 received normal horse serum (NS), and 5 were untreated. None of the dogs which were untreated or received NS passed urine after removal of their second kidney, none survived beyond 26 days after transplantation, and at autopsy the transplants were completely necrotic. The dogs treated with ALS survived for periods ranging from 27–79 days and six of them had evidence of graft function up to the day of death. The absolute lymphocyte count in the peripheral blood fell sharply after a single injection of ALS but animals which received repeated daily injections showed only a moderate degree of lymphopenia. It was concluded therefore that the immunosuppressive effect of ALS could not be accounted for simply by depletion of blood lymphocytes. The significance of the findings and their possible clinical applications are discussed.