Insulin-dependent diabetes mellitus (IDDM) is a disease characterized by the autoimmune destruction of the pancreatic beta-cells, which requires the expression of a number of immune-related genes including major histocompatibility complex proteins, cytokines, chemokines, and cytotoxic enzymes, many of which are regulated by the transcription factor, NFkappaB. Inhibition of the entire NFkappaB family of transcription factors may be harmful, as these factors are involved in many normal physiological processes. However, identifying and targeting specific NFkappaB subunits critical for the pathogenesis of disease may prove to be valuable in designing new therapeutic strategies. To assess the potential role of the NFkappaB subunit, p50, in the development of IDDM, mice with gene disruption for NFkappaB (p50) were investigated for susceptibility to IDDM. We found that p50-deficient mice were fully resistant against multiple low-dose streptozotocin-induced diabetes, a model of diabetes with a strong autoimmune component. The site of involvement of NFkappaB (p50) lies at an early, critical juncture of immune activation and proinflammatory mediator production, because: (1) isolated islets of Langerhans from NFkappaB (p50)-deficient mice were not protected from the islet dysfunction induced by in vitro application of proinflammatory cytokines; (2) p50-deficient mice were not resistant to diabetes induced by a single high dose of streptozotocin, a model with a large oxidant component and no autoimmune involvement; and (3) diabetes induced up-regulation of nitric oxide and interleukin-12 was blocked in the p50-deficient mice. Our data suggest that NFkappaB (p50) has an essential role in the development of autoimmune diabetes. Selective therapeutic blockade of this subunit may be beneficial in preventing IDDM.