The role of thymic functions in the development of insulin-dependent diabetes was investigated in athymic nude (nu/nu) mice and euthymic heterozygous (+/nu) littermates of BALB/c origin treated with streptozotocin. The injection of a single high dose of streptozotocin (200 mg/kg body weight) induced rapid and permanent hyperglycemia in both nu/nu and +/nu mice. In contrast, the injection of the same total dose divided into multiple "subdiabetogenic" doses (40 mg/kg per day for 5 consecutive days) caused the development of delayed but progressive hyperglycemia only in the thymuscompetent +/nu mice. Female mice of either genotype were significantly less susceptible to streptozotocin at both doses. Restoration of thymic immunity in nu/nu mice by thymus grafts also restored the susceptibility to the hyperglycemic effects of multiple low doses of streptozotocin. Moreover, splenic lymphocytes from +/nu mice previously made diabetic with the multiple low-dose injections of streptozotocin induced transient glucose intolerance in nu/nu mice. The ability of the diabetic spleen cells to transfer the diabetic state was abolished when the splenic lymphocytes were depleted of the T cells but not when they were depleted of B cells. These results provide direct proof that thymus-dependent functions play an obligatory etiologic role in the development of diabetes in mice treated with repeated subdiabetogenic doses of streptozotocii. These observations also add to the growing evidence that autoimmune amplification mechanisms may be critically involved in the etiology of juvenile-tnset diabetes mellitus in humans.There is increasing evidence that the development of the juvenile-type insulin-dependent diabetes mellitus is mediated by autoimmune processes. Pancreatic islets of children with newly diagnosed insulin-dependent diabetes frequently contain infiltrating mononuclear lymphocytes (1), and a large fraction of juvenile-onset diabetes patients have both circulating antibodies against the islet cells (2) and lymphocytes with cytolytic activity against human insulinoma cells in culture (3). In addition, epidemiologic studies in several countries have independently established that a number of alleles of the HLA gene complex, especially of the HLA-D locus, are associated with a significantly increased risk of this form of diabetes (reviewed in ref. 4).Evidence implicating autoimmune processes in the etiology of insulin-dependent diabetes comes also from experimental model systems utilizing laboratory rodents. In a series of experiments, Like and Rossini and their coworkers have shown that the induction of diabetes in mice and rats by the beta-cell toxin streptozotocin typically proceeds in two quite different pathways. When a single, sublethal, high dose of streptozotocin was administered to susceptible animals, rapid destruction of the islet beta cells ensued, followed almost immediately by a profound and permanent hyperglycemia (5). In contrast, the administration of the same total dose of streptozotocin, divided into m...