Nylon wool adherent, CD4+ T cells from the spleens of rats that have recovered from experimental autoimmune encephalomyelitis (EAE) inhibit the in vitro production of IFN-gamma, but not IL-2, by effector cells of EAE when cocultured in the presence of myelin basic protein Ag. When anti-transforming growth factor-beta (TGF-beta) antibodies are added to the co-cultures, IFN-gamma production is restored to normal levels. Irrelevant control antibodies have no effect. The same pattern of response was obtained with cells incubated in serum-free medium. In other experiments, purified TGF-beta was added to cultures of effector cells in the presence of antigen. TGF-beta inhibited the production of IFN-gamma by these cells in a dose-dependent manner, but had no apparent inhibitory effect on IL-2 production. Finally, supernatants from cultures containing effector cells and CD4+ suppressor cells plus Ag contained measurable amounts of TGF-beta, whereas supernatants from cultures of effector cells plus Ag contained no measurable amounts of TGF-beta. These results suggest that CD4+ Ts cells of EAE regulate effector cells of EAE through a mechanism that involves the secretion of TGF-beta and that the inhibitory function of this cytokine can be reversed with neutralizing antibodies directed against TGF-beta.