Immobilized antigen-antibody complexes are able to inhibit the mitogenic response of murine spleen cells to the B-cell mitogen 8-bromo- 3',5'-cyclic guanosine monophosphoric acid. This this inhibition is dependent on intact Fc fragments in the immobilized complexes. Soluble complexes do not mediate this inhibition. When lipopolysaccharide (lps) activation of B cells was studied, it was found that the mitogenic response was inhibited at all times tested between 2 and 7 days of culture. Also, the LPS-induced mitogenesis of nude spleen cells was inhibited by immobilized complexes, indicating that suppressor T cells probably play no significant role in the inhibition. Immobilized complexes inhibit polyclonal antibody responses in a serum-free system and in the presence of normal mouse serum, but are unable to inhibit in the presence of fetal calf serum (FCS). If nu/nu spleen cells are used, however, the FCS does not block the ability of the complexes to inhibit the polyclonal response. It is suggested that that antigen-antibody complexes under appropriate conditions may bind to B lymphocytes via their Fc receptors and trigger a central "off" signal which blocks proliferation and consequently antibody production.