In this study, we demonstrated that some V beta 6+, CD4+, Mls-1a-specific T cell clones had cytolytic activity when stimulated with anti-T cell receptor(TcR)/CD3 monoclonal antibodies (mAb), but not with targets expressing Mls-1a, although they produced lymphokines (interleukin 2 and interferon-gamma) in response to both types of stimuli. To examine the possibility that lack of cytolysis resulted from expression of the Mls-1a antigen on merely a fraction of splenic B blasts, we (a) used the B cell lymphoma LBB.3.4.16 and (b) measured esterase secretion which is generally concurrent with cytotoxic T lymphocyte (CTL) activity. The B cell lymphoma maximally stimulated the T cell clone for interferon-gamma production when responding and stimulating cells were incubated at a 1:1 ratio, but it was never killed by the Mls-1a-specific T cell clone unless TcR/CD3-specific mAb were added. Furthermore, a fivefold excess of the Mls-1a B cell lymphoma did not induce any secretion of esterase, which was observed only in the presence of the TcR/CD3-specific mAb. Comparison of the reactivity of two Mls-1a-specific T cell hybridomas expressing the same TcR at similar surface density, revealed both quantitative and qualitative differences between CD3-specific mAb and Mls stimulation of the hybridomas. A small quantitative difference in the sensitivity of hybridoma FJ22.5 to stimulation with V beta 6 or CD3-specific mAb resulted in a marked decrease in efficiency of stimulation by Mls-1a for interleukin 2 production and to inability to detect growth inhibition by Mls-expressing cells. A qualitative difference was observed when analyses of inositol phosphate production were performed under optimal conditions of stimulation of the highly responsive T cell hybridoma (FJ8.1): only stimulation with CD3-specific mAb, but not Mls-expressing cells, could induce detectable inositol phosphate production. Lack of cytolysis of Mls-1a class II-expressing B cells may have evolutionary significance in view of the recent mapping of Mls to mouse mammary tumor virus genes.