The immune response to oral Ag administration, including the development of oral tolerance, was explored with the use of OVA TCR-transgenic mice. Feeding high doses of OVA enhanced IFN-gamma production in the Peyer's patches, but induced tolerance in the peripheral lymphoid tissues marked by suppressed proliferative and cytokine responses. Systemic administration of Abs to IL-12 (anti-IL-12) simultaneous with Ag feeding modestly enhanced the degree of tolerance in the peripheral lymphoid tissues, as shown by increased suppression of proliferative responses after in vitro restimulation, and secondary responses in the popliteal lymph nodes following in vivo challenge and in vitro restimulation. Systemic anti-IL-12 treatment was associated with augmented TGF-beta production and T cell apoptosis in both Peyer's patches and peripheral lymphoid tissues. Cell mixing studies and proliferation assays in the presence of anti-TGF-beta provided evidence that the increased suppression of responses induced by anti-IL-12 was due primarily to the secretion of TGF-beta. These findings suggest that IL-12 negatively regulates two of the main mechanisms of oral tolerance, TGF-beta production and clonal deletion via apoptosis. in addition, they suggest that the combination of oral Ag feeding and systemic anti-IL-12 administration may be of benefit in the treatment of autoimmune diseases.