We have tested the effects of FK506 (FK), a new immunosuppressive agent, on a rat limb allograft model. Histoincompatible BN limb allografts were rejected in untreated F344 hosts within 11±1 days (mean ± SD) after operation. A single injection of 2 mg/kg, 10 mg/ kg, or 50 mg/kg of FK on the day of limb transplantation (day 0) significantly prolonged graft survival in a dose-dependent manner—i.e., mean limb survival times (MST) based on gross signs of skin rejection were 16±3 days, 51 ±6 days, or 104±17 days, respectively (P< 0.01). Delayed treatment with a single injection of 10 mg/kg of FK at when early signs of rejection were visible (day 7 or day 10) reversed the ongoing rejection. The MSTs in these groups were comparable to that of those treated with the same dosage of FK on day 0. The FK-induced unresponsiveness toward limb allografts was donor-specific because limb-allografted, FK-protected rats could not accept the skin grafts from a third-party donor. In the next set of experiments, rats were given a single administration of 10 mg/kg of FK on the day of limb allograft, followed by intermittent injections of 3 mg/kg of FK once a week. This regimen produced complete graft survival for more than 200 days, thoughPneumocyatis cariniipneumonia occurred in most of the recipients. These results represent the unique effects of FK in preventing or reversing the graft rejection and in inducing indefinite survival in this animal model of composite tissue allografts.