As rats were splenectomised up to 7 weeks before or at the time of receiving an (AS × August)F1 kidney allograft. Only 2 of 19 splenectomised recipients rejected their grafts within 12 days, and 14 of the 19 survived for more than 100 days. All 19 splenectomised recipients had a marked and often complete suppression of the lymphocytotoxic antibody response to the graft. In contrast, splenecotomy had no effect in suppressing the rejection of (AS × August)F1 renal allografts transplanted to AS recipients previously immunised with August lymphoid tissue, or in AS recipients of homozygous August renal allografts. Splenectomised recipients could be reconstituted with spleen cells from sensitised but not from normal AS rats. Splenectomy was found neither to augment nor diminish the effectiveness of passive enhancement of (AS × August)F1 or August kidneys in AS recipients. Splenectomy of AS recipients 2 or 4 days after an (AS × August)F1 renal allograft was as effective as pregraft splenectomy in suppressing rejection, but there was a deficiency of long survivors in a group splenectomised 1 day after grafting. A working hypothesis to explain these results is presented, and their clinical relevance is discussed.