The objective of this study was to determine whether mature thymic-derived T-lymphocytes were required for streptozotocin (SZ)-induced insulitis. C57BL/KsJ male mice were immunocrippled by thymectomy at 3 wk of age followed 1 wk later by lethal irradiation (1000 R) and hematopoietic reconstitution with syngeneic bone marrow (pretreated with anti-Thy 1.2 antiserum and complement to eliminate mature T-lymphocytes). As a control for the systemic effects of lethal irradiation itself, thymus-intact males were also irradiated and reconstituted with anti-Thy-1.2-treated marrow cells. This latter treatment resulted in a reconstitution of functional T-lymphocytes. Independent of the presence or absence of functional T-lymphocytes, irradiation extensively damaged the testes and produced at least a 50% reduction in plasma testosterone levels. In such effeminized males, the hyperglycemic response following 6 daily injections of SZ (35 mg/kg) was reduced in comparison to unirradiated males. Pancreatic insulin content was reduced 50% in both thymus-intact and thymectomized groups receiving lethal irradiation and SZ treatment; this correlated with histologic findings of small, beta-cell-depleted islets. Focal leukocytic infiltrates of the exocrine pancreas were induced by the irradiation. Streptozotocin-induced Insulitis was also observed regardless of the presence (in thymus-intact mice) or absence (in thymectomized mice) of phytohemagglutinin-responsive T-lymphocytes. Both groups exhibited intact B-lymphocyte function as measured by proliferative responsiveness to lipopolysaccharide. Severe immunosuppression of both T- and B-lymphocyte function was produced by subcutaneous injection of hydrocortisone into thymectomized mice 48 h prior to initiation of SZ treatments. This treatment prevented SZ-induced beta-cell necrosis and eliminated lymphocytic infiltrates in the endocrine and exocrine pancreas. We conclude that functional (mature) T-lymphocytes are not required to mediate the beta cytotoxicity of multiple low doses of SZ in inbred strains in which insulitis accompanies islet destruction. The ability of hydrocortisone to protect beta-cells from the direct cytotoxic action of SZ as well as to eliminate leukocytic infiltration in the pancreas would support the hypothesis that insulitis is a consequence of beta-cell destruction, in this model, rather than its cause.