The effect of FK506 was studied in sensitized and nonsensitized recipients using a rat cardiac allograft model. Heart grafts from Male ACI rats were heterotopically transplanted to Male LEW rats. The sensitized rat models were established by using donor type blood, admixed with immunoadjuvant (Adjuvant Complete Freund), seven days prior to transplantation. FK506 was administrated from day 0 to day 14 posttransplantation. The results showed that FK506 could prolong allograft survival in both sensitized and nonsensitized rat recipients in a dose-dependent manner, although, the dose required in sensitized recipients was higher than in nonsensitized recipients. The minimal effective dose of FK506 was 0.32 mg/kg/day in sensitized recipients and 0.1 mg/kg/day in nonsensitized recipients.