The effect of the immunosuppressant FK-506 on the development of diabetes in BB/Wor rats was investigated. Using a treatment schedule (25 micrograms i.m. from day 27 to 120), not associated with detectable general ill effects, this drug was found to completely inhibit the appearance hyperglycemia and to reduce the histological signs of pancreatic insulitis. The treatment was also able to reduce the percentages of Ia+ T-lymphocytes and to block the appearance of detectable serum levels of gamma interferon (IFN).