Tumor necrosis factor (TNF)-α and lymphotoxin (LT) α/β play multiple roles in the development and function of the immune system. This article focuses on three important aspects of the effects of these cytokines on the immune response and on autoimmunity. In several experimental systems (Jurkat T cells, murine T-cell hybridomas), TNF-α appears to cause a downregulation of signaling through the TCR, revealed by changes in calcium flux, activation of p21, p23 and ZAP70, and a decrease in nuclear activation of NF-κB. Previous and present results suggest that TNF-α interferes in some manner with signaling through the TCR, at a locus yet to be delineated. Transgenic expression of LTβR-Fc in nonobese diabetic (NOD) transgenic mice results in prevention of type 1 diabetes in NOD mice as long as the level of expression of the fusion protein (under the control of the cytomegalovirus promoter) remains above a level of 2–3 μg/ml. Once the expression levels of the fusion protein have dropped below this critical level, the diabetic process resumes and the animals become diabetic at 40–50 weeks of age, whereas nontransgenic littermates develop diabetes by 25–30 weeks of age. The paradoxical effects of neonatal TNF-α administration in NOD mice in increasing incidence of and hastening onset of type 1 diabetes, while neonatal anti-TNF administration completely prevents all signs of islet cell autoimmunity, are due partly to the low levels of CD4⁺CD25⁺ T cells in NOD mice. These low levels are reduced by a further 50% on neonatal administration of nontoxic levels of TNF-α. In contrast, neonatal administration of anti-TNF-α results in a dramatic increase in the levels of CD4⁺CD25⁺ regulatory T cells, to levels beyond those seen in wild-type untreated NOD mice. TNF-α and LTα/β thus have pleomorphic regulatory effects on the development and expression of autoimmunity.