This study concerns the effect of a 12-day cyclosporin A (CsA) treatment (50 mg per kg per day) on "autoimmune" diabetes induced by 5 low doses (40 mg per kg per day) of streptozotocin (SZ). The SZ-treatment period was initiated 4 days after initial administration of CsA. In young (45-day) CD-1 male mice, CsA enhanced hyperglycemia, hypoinsulinemia and beta-cell destruction following a multiple low-dosage SZ. Moreover, CsA did not prevent development of insulitis induced concomitantly by SZ. Similarly, CsA enhanced the "toxic" diabetes produced by a single high dose (160 mg/kg) of SZ. Furthermore, in the absence of SZ, CsA alone induced glucose intolerance, associated with beta-cell degranulation and high pancreatic CsA content. The enhancement of SZ-induced diabetes by CsA may thus be due to toxicity of the immunosuppressive agent for pancreatic beta cells. This side effect is noteworthy because CsA is currently being used in the therapy of human insulin-dependent diabetes.