The mixed agglutination test with cell cultures was used for demonstration of humoral antibodies accompanying renal homograft. Negative results were obtained with sera of 11 patients carrying renal grafts with intact circulation. In contrast, antibodies were demonstrated in the serum of a patient who was maintained on dialysis after surgical removal of the rejected kidney graft. A hypothesis was advanced that humoral antibodies formed by patients with renal homografts are removed from the blood circulation by the graft. This was substantiated by experiments on dogs. Renal homografts in dogs elicited formation of humoral antibodies which were detectable a few days after cessation of the blood flow through the renal artery. Following the second kidney graft from the same donor, the antibodies immediately disappeared but they reappeared again a few days after removal of the second graft.