The synthetic polymers L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT) and L-glutamic acid50-L-tyrosine50 (GT) stimulate specific suppressor T cells in certain strains of mice. Extracts from these T cells contain factors (TsF) that inhibit GAT- or GT-specific antibody responses by normal spleen cells or proliferative responses by primed T cells. We constructed T cell hybridomas that constitutively produce GAT-TsF or GT-TsF, which functionally and serologically are identical to factors extracted from suppressor T cells. In this report we demonstrate that monoclonal GT-TsF can induce specific unresponsiveness in vivo or in vitro and that this unresponsiveness is due to development of second-order antigen-specific suppressor T cells. T cell hybridomas were constructed by fusion of BW5147 with GT-TsF1 induced second-order suppressor T cells and clones that produced suppressor factor (GT-TsF2) were isolated and characterized. GT-TsF2 differs from the GT-TsF1 used to induce it in that GT-TsF1 acts across allogeneic barriers whereas GT-TsF2 does not. This restriction is controlled by genes in the H-2 gene complex and maps to the I-J subregion. GT-TsF2 is antigen-specific in suppressive activity and also in its antigen-binding site(s). Thus, GT-TsF2 closely resembles the carrier-specific, I-J+, genetically restricted factor described by Tada and his colleagues. Because GT-TsF2 was induced by GT-TsF1, we suggest cells producing GT-TsF1 are an early cell in the pathway of suppression, and that this cell is required for the activation of antigen-specific, MHC-restricted TsF.