We have previously described a monoclonal antibody, B16G, which has been found to be specific for T-cell derived suppressor factors (TsF). B16G has been shown to react with T-suppressor cells, TsF in the spleen of normal or tumor-bearing mice, the TsF produced by tumour-specific, or hapten-specific T-cell hybridomas, and with polyclonal whole human TsF isolated from tonsillar tissue. This pan-reactivity inherent to the B16G antibody suggests that it recognizes some common, shared epitope of the TsF molecule. In this study, we have used B16G as a probe to isolate a TsF-producing T-cell hybridoma, S-50, from CBA mice (H-2k) that is specific for the Lupus-associated antigen, RNP-Sm. The TsF bound specifically to RNP-Sm and inhibited the production of anti-RNP-Sm antibody cell cultures from MRL-lpr mice. SDS-PAGE analysis of purified S-50 TsF revealed a B16G-reactive band with a molecular weight of43kd.