SUMMARYRenal transplantation was performed between two strains of rat (AS and AS2) known to differ at the major histocompatibility locus, and the results were compared to those of skin grafting. Skin grafts exchanged between the strains were rejected rapidly, as were AS2 renal transplants made to rats of the AS strain. Transplantation of AS kidneys to AS2 rats gave very different results: 70% of the recipients survived for more than 50 days with good renal function and minimal evidence of rejection in their transplants. Shorter survival times were seen, however, when AS kidneys were transplanted to AS2 rats previously sensitised toward the renal donor. Long surviving AS2 recipients of AS renal allografts accepted subsequent AS skin grafts and it has been suggested that these AS2 recipients might have become immunologically tolerant of the donor strain.