In the light of recent attempts to treat newly-diagnosed Type 1 (insulin-dependent) diabetic patients with cyclosporin A, and reports suggesting an impaired glucose tolerance following immunosuppression therapy with cyclosporin A, we investigated the long-term effects of cyclosporin A on islet beta-cell morphology and function in vitro. Collagenase-isolated mouse pancreatic islets were cultured free-floating for 7 days in medium RPMI 1640 + 10% calf serum in the presence of cyclosporin A (0.1 or 1.0 mg/l). Islets cultured in the presence of the higher cyclosporin A concentration had impaired islet proinsulin biosynthesis and insulin release when challenged with high glucose concentration. Moreover, the insulin content of the drug-exposed islets was decreased and so was the rate of DNA synthesis. The glucose oxidation and respiratory rates, however, remained unaffected, suggesting that the impaired insulin production was not a result of defective oxidative metabolism. There were no changes in the ultrastructure or phospholipid biosynthesis of the islets after the drug treatment. These data indicate that cyclosporin A affects islets in culture, the clinical implications of which are so far difficult to assess. The inhibitory effect of cyclosporin A on islet cell DNA synthesis must nevertheless be considered in attempts to ameliorate Type 1 (insulin-dependent) diabetes, and when grating islet cells in numbers primarily insufficient to cure the recipient.