Suppressor of cytokine signaling-1 (SOCS-1) is a negative regulator of the Jak-STAT (signal transducer and activator of transcription cytokine) signaling pathway but may also regulate other pathways. At least in vitro, SOCS-1 inhibits the action of multiple cytokines. By studying the effects of SOCS-1 deficiency, we investigated whether SOCS-1 is involved in preventing cytokine-induced death of pancreatic islet cells, a potential mechanism of insulin deficiency in autoimmune diabetes. Tumor necrosis factor (TNF) + interferon-gamma (IFNgamma) was more potent at inducing cell death in SOCS-1-/- islets than in wild type. Individually, these cytokines did not induce cell death. The titration of the two cytokines suggested that this increased cell death was because of hypersensitivity to TNF. Interleukin-1 + IFNgamma induced the same level of cell death in SOCS-1-/- and wild-type islets, suggesting that the sensitivity of islets to IFNgamma or interleukin-1-mediated cytotoxicity is not affected by SOCS-1 deficiency. Additionally, SOCS-1-/- beta cells were responsive to lower concentrations of TNF measured by class I major histocompatibility complex up-regulation. The TNF + IFNgamma damage of islets was mediated by inducible nitric-oxide synthase (iNOS), and increased iNOS expression and nitric oxide production were found in SOCS-1-/- islets following cytokine treatment. A further analysis revealed that SOCS-1 deficiency results in augmented TNF signaling via the p38 mitogen-activated protein kinase pathway but not NFkappaB or c-Jun N-terminal kinase pathways. Increased p38 signaling may be responsible for the increased iNOS expression in SOCS-1-/- islets. Therefore, these findings provide evidence that physiological levels of SOCS-1 negatively regulate TNF signaling.