Thirty-nine (LEW × BN)F1kidneys were transplanted to LEW rats. Twenty-four untreated recipients survived for a mean time of 16.1 ± 1.7 days (group 1). Fifteen recipients received 4 ml of antilymphocytic serum per rat (group 3). In the last group 10 recipients survived for more than 4 months. The spleen cells of these permanently surviving 10 rats were obtained by splenectomy and used in a graft-versus-host assay, and this assay showed that the reactivity of these cells was normal. Following splenectomy the animals were given an (LEW × BN)F1skin allograft, followed 18 days later by a second. After another 18 days (LEW × Buf)F1“third party” skin allografts were transplanted to the same animals. Animals of group 2 rejected their first grafts with a mean survival time of 12.2 ± 1.2 days, whereas the second grafts were rejected normally as were the third party grafts. Attempts were made to detect lymphocytotoxic antibodies and haemagglutinins before and after the transplantation of skin grafts and none could be found up to day 53. The sera of group 2 inhibited allorosette formation by 38%. This serum-blocking factor was donor specific. It is probable that the survival of the kidney transplants following antilymphocytic serum treatment was brought about by the development of blocking antibodies.