Expression of IL-12Rs is one important checkpoint for Th1 development. BALB/c DO11.10 CD4+ T cells stimulated by Ag in neutral conditions lose expression of the IL-12R beta 2 subunit and become unresponsive to IL-12. In contrast, B10.D2 or F1 (BALB/c x B10.D2) DO11.10 CD4+ T cells maintain IL-12R beta 2 expression when stimulated similarly. Here we show that the loss of IL-12 responsiveness by BALB/c T cells involves the action of endogenous TGF-beta. BALB/c T cells stimulated in the presence of anti-TGF-beta specifically maintain IL-12 responsiveness, express IL-12R beta 2 mRNA, and can stimulate nitric oxide production in peritoneal exudate cells. Low concentrations of TGF-beta added exogenously during primary activation of B10.D2 or F1 T cells significantly inhibit their development of IL-12 responsiveness. These effects of anti-TGF-beta are dependent on endogenous IFN-gamma and are inhibited by exogenously added IL-4. Thus, at least one effect of TGF-beta on Th1/Th2 development may be the attenuation of IL-12R beta 2 expression.