Of several immunosuppressive drugs studied, procarbazine hydrochloride (PCH) alone produced a very marked synergy with antilymphocyte serum (ALS) in prolonging the survival of H-2-incompatible murine A/Jax skin allografts in CBA males. Nontoxic doses of the drug and ALS were administered on alternate days in the first postoperative week. When donor strain liver extract was administered i.v. 16 days before skin transplantation, followed by the same regimen of PCH and ALS, a highly specific and long-lasting unresponsiveness was induced in 70 to 85% of CBA male recipients. The addition of other drugs to this system after completion of the PCH-ALS treatment was either of no benefit or counterproductive, as was the inclusion of Bordetella pertussis vaccine 2 days before skin transplantation. Lymphocytes from unresponsive mice were normally reactive to donor antigens in vitro and in a graft-versus-host assay. No functional antibodies could be detected and the distribution of donor strain lymphocytes injected into unresponsive mice was normal. Unresponsiveness was, however, transferable to ALS-treated syngeneic recipients with purified splenic T cells, indicating that these cells have a central role in the mediation of this unresponsiveness. Induction of unresponsiveness depends on the precise temporal relationship between antigen, PCH and ALS administration, and skin transplantation. It has been shown to work in several other strain combinations but not when the recipients were A/Jax, and results were less good in female recipients. The implications for clinical transplantation are discussed.