Among female nonobese diabetic mice, ketotic insulin-dependent diabetes mellitus (IDDM) develops spontaneously in 80% between 12 and 26 weeks of age. This condition resembles human type I diabetes. IDDM developed in 0 of 11 (0%) mice prophylactically treated with 10 mg of cyclosporine A (CyA) per kg s.c. every 4th day from 8 to 26 weeks of age; 8 of 10 (80%) of sex- and age-matched controls developed IDDM; 2 of 8 (25%) followed up to 5 months beyond the time of drug administration developed IDDM. The distribution of specific radioactivity ([3H]CyA) was used to calculate the concentrations of CyA in serum, blood cells and various organs. Serum values of CyA produced by radioimmunoassay were higher than those estimated by the [3H]CyA method. Pancreata of CyA-treated mice were histologically normal. Pancreata of control mice showed lymphocytic infiltration of the islets of Langerhans. Neither hepatotoxicity nor nephrotoxicity assessed by biochemical and histological data was detectable in CyA-treated mice. The insulin secretory capacity of trypan blue viable functional pancreatic islets isolated post-treatment; was significantly lower in controls than in CyA-treated mice; islet content of insulin was not statistically different between controls and CyA-treated mice. We conclude that low nontoxic doses of CyA abrogate completely the development of diabetes in the female nonobese diabetic mouse and abolish lymphocytic infiltration of the islets of Langerhans against which there is autoimmunity. The effect of CyA persists well past the duration of therapy.