The induction of human T cell proliferation by antibodies that cross-link T3 antigens is dependent on functional interactions of anti-T3 antibodies with monocyte Fc receptors. In this report, we used a panel of anti-T3 antibodies of differing heavy chain isotype and a variety of other monoclonal antibodies to analyze several features of the antibody-mediated interactions between T cells and monocytes that are required for mitogenesis. Whereas three IgG2a anti-T3 antibodies were mitogenic for cells from all individuals, IgM and IgG2b anti-T3 antibodies did not induce T cell proliferation in any donor and could block the proliferative responses induced by other mitogenic anti-T3 antibodies. Dose-response analyses with four IgG1 anti-T3 antibodies demonstrated donor heterogeneity as reported by other investigators. However, in contrast to these previous reports, high concentrations of IgG1 anti-T3 antibodies were found to be mitogenic for all donors, indicating that this heterogeneity is based on relative rather than absolute defects in low responder monocytes. Cell mixing experiments in which monocytes from two different low responder donors were co-cultured with T cells and IgG1 anti-T3 antibodies did not identify any complementary defects, suggesting that the low responder phenotype results from a relatively restricted polymorphism. To assess the nature of the signals required for inducing T cell proliferation, nonmitogenic anti-T3 antibodies were co-cultured with other pan-T cell antibodies having the IgG2a isotype. The combination of signals from T3 antigen cross-linkage and those independently generated by other IgG2a antibodies bound to monocyte Fc receptors did not induce T cell proliferation. Hence, it appears that the T3 antigen or closely associated structures must be clustered at the monocyte membrane for mitogenesis. Finally, in competitive inhibition experiments, the isotype specificity of monocyte Fc receptors involved in the induction of T cell proliferation was examined. Two distinct Fc receptor sites, one that binds murine IgG2a and IgG3 antibodies and a second that binds murine IgG1 antibodies, were identified. Murine IgM or IgG2b did not appear to bind either of these receptor sites. Taken together, these data indicate that human monocytes have two distinct Fc receptor sites, which must specifically and directly interact with T cell-bound anti-T3 antibodies for mitogenesis.