We have previously shown that direct ultraviolet (UV) irradiation of islets can reduce their immunogenicity without alteration of their endocrine function and effect prolonged survival of islet allografts in the Lewis-to-ACI strain combination without the use of immunosuppression. This study extends that work to investigate the survival of UV-irradiated Wistar-Furth islets in streptozotocin-diabetic Lewis rats, in which case the recipient is a relatively "high responder" to W/F alloantigens. Lewis recipients of 24-hr cultured W/F islets uniformly rejected islet allografts within one week of transplantation while the additional immunosuppression with cyclosporine (CsA) at 15 or 30 mg/kg on days 0, +1, and +2 was ineffective in prolonging islet allograft survival. Wistar-Furth islets, which were UV-irradiated at 850-900 J/m2 and cultured for 24 hr prior to transplantation, did not survive any longer than those in control animals receiving untreated islets (MST 5.5 +/- 1 day). When UV irradiation of islets was combined with recipient peritransplant treatment with CsA at 15 mg/kg on days 0, +1, and +2 islet allograft survival was markedly prolonged (MST of 18 +/- 4.1 days). Treatment of diabetic recipients of UV irradiated islets with an increased dose of CsA (30 mg/kg) during the same peri-transplant period (0, +1, +2 days) resulted in 100% islet allograft survival beyond 120 days. This data demonstrate the effectiveness and synergism between the use of the pretransplant UV irradiation of islet allograft and the peritransplant immunosuppression of the recipient with CsA in inducing prolonged islet allograft survival in "high responder" recipients, in which the singular use of either modality may be ineffective.