The effect of FK-506 on allograft survival in unsensitized recipients has been established in several animal experimental models. In this present study, we investigated the immunosuppressive capacity of FK-506, in comparison with ciclosporin (Cs) and 15-deoxyspergualin (DSG), on heart allografts in presensitized rats. Heart grafts from male ACI rats were heterotopically transplanted to male LEW rats. All recipient rats were sensitized with donor-type whole blood admixed with immunoadjuvant (adjuvant complete Freund) 7 days prior to transplantation. FK-506, Cs and DSG were administered from day 0 to day 14 posttransplantation. The results showed that both FK-506 and Cs significantly prolonged heart allograft survival of presensitized recipients in a dose-dependent manner. The minimum effective dose in the treatment was less for FK-506 than for Cs. However, DSG showed almost no effect of prolongation in this experiment.