The effects of cyclosporin, an immunosuppressive agent used in diabetic and nondiabetic patients, on in vitro glucose-induced insulin secretion were evaluated in isolated islets and in a glucose-responsive clonal pcell line (HIT cells). Cyclosporin inhibited glucose-induced insulin secretion in a dose-response manner at concentrations commonly found in human blood. With isolated islets, four time periods (0–5, 5–15, 15–30, and 30–60 min) were examined after stimulation with 300 mg/dl glucose. Inhibition of insulin secretion by cyclosporin was evident by 5–15 min as well as during later times with progressively smaller drug concentrations. With HIT cells, longer-term effects were examined after 16 h of incubation with various drug concentrations. Inhibition of insulin secretion was again observed, and these inhibitory effects were not reversed by drug washout. A cyclosporin concentration of 0.1 μg/ml, which is a therapeutic blood level in humans, was sufficient to inhibit insulin secretion in both in vitro models. Patients using this drug should be carefully monitored for signs of deficient insulin secretion.