We have previously demonstrated that a 12-day course of cyclosporine A (CsA) leads to the induction of tolerance to renal allografts in 100% of recipients selectively mismatched at class I for both haplotypes, and in 71% of recipients selectively mismatched at class II for both haplotypes, but in 0% of recipients mismatched for two haplotypes at both class I and class II. We have postulated that the mechanism by which tolerance is induced may therefore require matching for either class I or class II antigens. One might predict from this hypothesis that tolerance would also be induced in donor-recipient combinations sharing one full haplotype (eg, AC→ AD), which mimics the clinically relevant transplant combination of parent to offspring. We have therefore investigated the effects of the CsA regimen on renal transplants in this combination. Without immunosuppression, such kidney allografts were uniformly rejected (n= 12; 10.6±2.4 days). In contrast, a course of CsA (10–13 mg/kg/day) during the first 12 postoperative days induced long-term acceptance of the allograft in 67%(46) of recipients. Some acceptor animals also showed specific unresponsiveness to donor antigens as measured by in vitro assays and by failure to develop anti-donor antibodies. Tolerance was confirmed in four of these animals by failure to reject a second transplant SLA-matched to the first kidney donor without additional immunosuppression. These results suggest the feasibility of inducing specific tolerance across a singlehaplotype mismatch in the majority of the cases, which could have clinical implications for livingrelated transplants.