Diabetes-prone (DP) BB rats spontaneously develop a hyperglycaemic condition which closely resembles human insulin-dependent diabetes mellitus (IDDM), both in terms of clinical and histological features. The incidence of IDDM was significantly reduced when these animals were treated with 2 or 4 mg fusidic acid (FA)/day i.m. from day 30 to day 120 of age. In addition, the mean insulitis score was significantly diminished in the animals treated with FA compared to both vehicle-treated and untreated controls. Finally, 2 mg/day of FA i.m. prevented cell proliferation and interferon-gamma secretion from peripheral blood mononuclear cells upon ex vivo stimulation with concanavalin A. The capacity of FA to substantially reduce the incidence of autoimmune diabetes in a well-known animal model of human IDDM supports previous observations regarding the immunosuppressive properties of FA and its potential use in the treatment of human autoimmune diabetes.