Transforming growth factor-beta 1 (TGF-beta 1) is a cytokine with complex immunomodulatory effects including the ability to inhibit the onset or severity of autoimmune disease. This study was designed to test the possibility that one mechanism by which TGF-beta 1 exerts its immunosuppressive effects is by inducing antigen (Ag)-specific unresponsiveness in CD4+ cells. TGF-beta 1 was shown here to inhibit the Ag-specific proliferation of naive CD4+ cells from T cell receptor (TCR) transgenic mice. More importantly, the naive CD4+ cells exposed to TGF-beta 1 and Ag, but not to TGF-beta 1 alone, in primary cultures were unable to proliferate or secrete IL-2 in response to a subsequent Ag challenge following removal of TGF-beta 1 from the cultures. Anti-CD28 mAb partially blocked the Ag-specific inactivation induced by TGF-beta 1 in naive CD4+ cells. The inhibitory effects of TGF-beta 1 on CD4+ cells are not mediated by alterations in APC costimulation since TGF-beta 1 did not inhibit the Ag-induced expression of MHC class II molecules, CD80 or CD86 on splenic APC. Taken together, the results suggest that the immunosuppressive activities of TGF-beta 1 encompass direct induction of Ag-specific unresponsiveness in naive CD4+ cells.