Equine antithymocyte globulin combined with azathioprine and prednisone as immunosuppressive therapy in 50 transplant recipients prolonged allograft survival and seemed to modify the severity of rejection episodes. Although 9 patients died from a variety of causes, only 3 kidneys were lost to rejection, one of which was hyperacute. There were no serious untoward hematologic or systemic effects caused by the ATG, and all patients completed the course of therapy. Infection, a serious and frequent complication of transplant patients, was encountered no more often than in other transplant series not using antilymphocyte globulin. globulin. The data pertaining to the clinical value of ATG, although suggestive in terms of its immunosuppressive effects, are still not conclusive; and a definitive answer to this question awaits further evaluation in a series of cadaveric recipients in a randomized double blind study.