Although class II antigens encoded by genes in the major histocompatibility complex (MHC) are important as recognition structures for immunoregulatory cell interactions, the precise functional role of these molecules in the biological responses of B lymphocytes is unknown. In the studies described here, we have examined the effects of six monoclonal antibodies reactive with human class II MHC antigens on B cell activation and proliferation. Peripheral blood IgM+ B cells purified by fluorescence-activated cell sorter (FACS) techniques were stimulated with anti-mu antibodies, protein A-bearing Staphylococcus aureus (SAC), or in T cell-dependent activation cultures. The B cell proliferative responses induced by these stimuli were inhibited 68 to 90% by low concentrations (1 to 5 micrograms/ml) of antibodies reactive with class II MHC antigens. Antibodies specific for DR and DQ antigens were both effective inhibitors of B cell proliferation. This inhibition was not due to the binding of antibody to B cell Fc-IgG receptors, because IgM and IgG anti-class II antibodies were equally potent as inhibitors. When responses of B cells fractionated on the basis of cell size by forward angle light scatter were analyzed, anti-DR and anti-DQ antibodies inhibited the proliferation of small, resting IgM+ cells induced by T-independent as well as T-dependent stimuli. Activation-dependent increases in B cell size and RNA synthesis were similarly inhibited. In contrast, the responses of large B cells (that had been preactivated in vivo) to T cell-derived B cell growth factors were not affected by anti-class II antibodies. These data suggest that class II MHC molecules do not serve merely as cellular interaction structures but also directly participate in early events of the B cell activation cascade that precede cell enlargement or increased RNA synthesis. After activation and expression of receptors for growth factors, however, B cell class II MHC antigens no longer mediate signals required for mitogenesis.