The T4 molecule has been identified as a marker of human T cell differentiation, but the function of this molecule remains to be defined. We have investigated its possible functional involvement in T cell proliferative responses to class II HLA antigens encoded by the recently described SB locus. The responses of SB-primed cells (specific for each of four different SB antigens) were studied with the use of two proliferation-inducing stimuli, SB antigen or TCGF. The proliferative responses to both stimuli were found to be mediated by T4+, T8- cells. Monoclonal antibodies against some epitopes on the T4 molecule (OKT4A and OKT4B) substantially blocked antigen-stimulated proliferative responses; antibodies against other epitopes of the T4 molecule (OKT4, T4C, T4D) blocked less well. Inhibition of SB-specific proliferation by antibodies to the T4 molecule was maximal only when the antibodies were incubated with the responder cells before the addition of stimulator cells. Proliferative responses of SB-primed cells stimulated with TCGF alone were not inhibited by any of the OKT4-related antibodies, but were completely inhibited by the anti-Tac monoclonal antibody, which reacts with the TCGF receptor. These results lend further support for the hypothesis that the T4 molecule is involved in T cell recognition of and/or activation by class II HLA antigens. We suggest that 1) the T4 molecule binds a nonpolymorphic epitope on class II HLA molecules, and 2) this interaction may facilitate, but not be an obligate requirement for, T cell activation by class II antigens.