FK506 is a new immunosuppressive agent which has been found more potent than cyclosporine based on the dosage. FK506 was examined here for its effect on the development of uveitis in primates immunized with S-antigen. FK506 successfully inhibited uveitis in monkeys, even when administered three weeks after the first immunization, at the time when the immunopathogenic mechanism of uveitis is assumed to be developed. All four monkeys injected with 0.5 mg/kg/day of FK506 did not develop uveitis, 2 out of 4 treated with the 0.25 mg and 3 out of 4 of those receiving the 0.125 mg also did not develop disease. FK506 suppressed to some extent the cellular and humoral immune responses to S-antigen. The main side effect of FK506 was weight loss. We consider that this drug may be considered as a new potential therapeutic agent for immune-mediated ocular disease in humans.