We investigated the preventive effect of the immunosuppressive agent FK506 on autoimmune insulitis in nonobese diabetic mice. The mice were given FK506 in a dose of 1.0 mg/kg, every other day, from age 2 to 12, 2 to 6, and 4 to 12 weeks, respectively; after which, the incidence of insulitis and overt diabetes was monitored. Effects of FK506 on immune reactions to beta cells were also investigated by using both syngeneic and allogeneic islet transplants. Treatment with FK506 in mice from age 2 weeks prevented completely the onset of overt diabetes, and the incidence of insulitis was reduced to less than 10% at age 30 weeks. Treatment of mice with FK506 from age 4 weeks was less effective in preventing insulitis and the onset of diabetes. In case of islet transplantation, FK506 treatment of NOD mice from age 2 to 6 weeks prevented autoimmune responses both in syngeneic islets and in allogeneic islets, which share the same H-2 antigen with the nonobese diabetic mouse. These results also indicate that the recognition of islet antigens and the generation of autoimmune-reactive T lymphocytes start between 2 and 4 weeks of age, and FK506 prevents an autoimmune reaction.