Nonobese diabetic (NOD) is an inbred mouse strain susceptible to development of T cell-mediated autoimmune diabetes. The strain is characterized by high percentages of T lymphocytes in lymphoid organs. The syngeneic mixed lymphocyte reaction (SMLR), a T cell response to self MHC class II Ag, is reportedly involved in the generation of a number of immunoregulatory cells, including suppressor inducers. A severely depressed SMLR characteristic of certain other autoimmune strains was found in NOD but not in nonautoimmune SWR/Bm mice. Moreover, IL-2 produced by NOD T cells at day 6 in an SMLR was at least one hundredfold reduced compared with SWR, and NOD T cells harvested from an SMLR at day 6 were functionally defective when tested for ability to induce suppression of an allogeneic MLR. However, functionally competent suppressor T cells were generated in NOD splenic leukocyte cultures in response to Con A, and IL-2 release from these was equivalent to that released by Con A-stimulated SWR splenocytes. A deficiency in cytokine release was not limited to IL-2, because peritoneal exudate cells from NOD exhibited a greatly diminished sensitivity to LPS-stimulated IL-1 release in comparison to SWR mice. IL-2 supplementation both in vitro and in vivo restored the ability of NOD T cells to respond in a SMLR, with production of cells capable of inducing suppression. Like SMLR-activated T cells from untreated SWR controls, SMLR blasts from IL-2-treated NOD mice were enriched for the L3T4 phenotype. IL-1 supplementation in vitro resulted in partial restoration of T suppressor activation in a SMLR. The depressed SMLR exhibited by NOD mice was apparently a stimulator cell dysfunction, because NOD stimulator cells failed to activate T cells from (SWR x NOD)F1 mice, whereas stimulators from SWR or F1 mice were capable of doing so. Collectively, these results suggest a defect in suppressor cell activation rather than an absence of this immunoregulatory cell population.