In the present study, we define the relation between TGF-β and IL-10 in the regulation of the Th1-mediated inflammation occurring in trinitrobenzene sulfonic acid (TNBS)-colitis. In initial studies, we showed that the feeding of trinitrophenol-haptenated colonic protein to SJL/J mice induces CD4+ regulatory T cells that transfer protection from induction of TNBS-colitis, and that such protection correlates with cells producing TGF-β, not IL-10. Further studies in which SJL/J mice were fed haptenated colonic protein, and then administered either anti-TGF-β or anti-IL-10 at the time of subsequent TNBS administration per rectum, showed that while both Abs abolished protection, anti-TGF-β administration prevented TGF-β secretion, but left IL-10 secretion intact; whereas anti-IL-10 administration prevented both TGF-β secretion and IL-10 secretion. Thus, it appeared that the protective effect of IL-10 was an indirect consequence of its effect on TGF-β secretion. To establish this point further, we conducted adoptive transfer studies and showed that anti-IL-10 administration had no effect on induction of TGF-β producing T cells in donor mice. However, it did inhibit their subsequent expansion in recipient mice, probably by regulating the magnitude of the Th1 T cell response which would otherwise inhibit the TGF-β response. Therefore, these studies suggest that TGF-β production is a primary mechanism of counter-regulation of Th1 T cell-mediated mucosal inflammation, and that IL-10 is necessary as a secondary factor that facilitates TGF-β production.