Phagocytosis mediated by IgM and C3 receptors was studied by in vitro experiments with peritoneal macrophages from mice treated with repeated i.p. injections of glycogen. The immune complexes were sheep red cells sensitized with mouse IgM antibody or antibody and complement. Macrophages from the inflammatory exudate induced by glycogen were able to phagocytize immune complexes of red cells and IgM antibodies in contrast to normal macrophages, which can only bind to these complexes. Evidence was also found for phagocytosis mediated by interaction with C3 receptors in the macrophages from the glycogen-treated animals. The implications of these findings in the mechanism of phagocytosis and the possible relevance of the observed changes in the function of macrophage receptors for the afferent and efferent branches of immune response are discussed.