The fine specificity of anti-idiotypic, effector-phase suppressor T cells (Ts2) induced by the intravenous injection of syngeneic spleen cells covalently coupled with the 4-hydroxy-3-nitrophenyl acetyl (NP) hapten was studied in an in vitro plaque-forming cell system. By comparing the ability of these suppressor cells to bind monoclonal anti-NP antibodies that express different levels of serologically detected NPb idiotypic determinants, it was shown that anti-idiotypic suppressor T cells do not recognize the predominant NPb idiotypic determinants that are defined by serologic analysis. The implications for the possible expression and/or recognition of different sets of idiotypic determinants on T and B cells are discussed.