Nineteen patients about to undergo renal homotransplantation were skin-tested with a battery of five antigens. Seven patients had a total of nine positive delayed-type reactions. Three days after daily treatment with an antilymphocyte preparation was started, antigens which had given a positive reaction were reinjected. No other immunosuppressive agents were being used at this time. Readings 2 days later showed that the skin reactions were negative in all cases.A great deal of interest in antilymphocyte sera (ALS) has arisen because of its ability to act as an immunosuppressive agent and to prolong the survival of homografts in rats (10), mice (3, 5), dogs (7), and man (7). A recent symposium has dealt with this topic (9). Its mode of action is not known, although several possibilities include (1) prevention of recognition of antigen (“blindfolding”), (2) “sterile activation” of lymphoid cells so they are incapable of responding to any antigen, and (3) lymphocytolysis(3, 4).Because the mechanism of graft immunity resembles that of delayed hypersensitivity, we thought it of interest to test the effect of ALS on delayed hypersensitivity reactions in uremie patients before kidney homotransplantation.