AbstractBB rats spontaneously develop an insulin dependent diabetes which resembles in many features human type I diabetes. We have tested the effect of the immunomodulatory drug Ciamexone, a 2-cyan-aziridine-derivative, on the development of diabetes in BB rats. Ciamexone was given once daily during 6 days per week beginning with the age of 42 or 50 days up to 120 days. For comparison cyclosporin A (10 mg/kg) was applied following the same protocol. At 1 mg/kg ciamexone administration led to complete prevention of diabetes in females but was not beneficial in males. At 10 mg/kg the drug caused significant suppression of diabetes development in males but more pronounced in females. Both, a reduction of the incidence of diabetes and a delay in the onset of hyperglycaemia was observed only in females. After administration of cyclosporin A none of the animals developed diabetes.Ciamexone treatment did not affect granulocyte and lymphocyte counts and subsets in the peripheral blood except for a tendency to suppress eosinophilia. The growth of animals was not retarded. It is concluded that ciamexone seems to influence the autoimmune state of the BB rat resulting in partial suppression of the disease.