Repeated adminsitration of monoclonal antibodies (mAb) directed againt the CD4 lymphocyte receptor may induce specific, long-lasting unresponsiveness to fully MHC-mismathed cardiac allografts in rats without additional immunosuppression. We assessed the effect of a single dose of murine anti-rat depleting anti-CD4 mAb (OX-38) on allograft survival in high- and low-responder rat strain combinations. Isogenic strains of DA (RT1 av1), PVG (RT1 c), AUG (RT1 c), and WF (RT1 u) rats were used. Recipients in antibody treated groups were given one dose of 5 mg/kg OX-38 mAb on the day of transplant, a dose which was shown to effectively deplete (or block) circulating CD4 + T cells. Other groups were treated for 10 days with cyclosporin A (CsA) and/or Linomide, a novel immunomodulator, which is the first compound able to fully eliminate the effect of CsA in the rat cardiac allograft model. The DA strain was identified as a low-responder to the allogeneic haplotype RT1 c (PVG or AUG), but not to RT1 u (WF), and developed true tolerance following RT1 c grafting and OX-38 or low-dose CsA (5 mg/kg) induction, as verified by the response to retransplantation of a graft from the same donor strain or a third-party challenge. PVG recipients of DA grafts were characterized by high response and only modest (OX-38; median 9.5 days) or moderate (CsA; 23.5 days) prolongation of graft survival. Contrasting graft survival results were obtained in the low-responder combination, either very early rejection (at 10 days) or permanent graft survival (> 100 days). Linomide challenge affected CsA treatment in the high-responder combination but not tolerance induction in the low-responder combination, or the effect of OX-38. It was concluded that in rat heart transplantation a single-dose anti-CD4 mAb therapy may induce permanent donor-specific unresponsiveness in a low-responder strain combination, and that anti-CD4 mAb seems to be unique among immunosuppressive agents while being resistent to challenge by Linomide.