Purified samples of IgM free of IgG and alpha 2-macroglobulin contaminants were obtained from mice sera. Resident peritoneal mouse macrophages poorly phagocytose sheep red blood cells (E) sensitized with purified mouse IgM anti-E. Conversely 62.5% of thioglycollate-elicited macrophages and 10.11% of BCG stimulated peritoneal cells phagocytose IgM sensitized sheep red blood cells. The monomeric and polymeric forms of IgM blocked almost completely the phagocytosis of erythrocytes opsonized with homologous IgM molecules. These data clarifies the controversy concerning the homologous IgM receptor in mouse macrophages and further suggest that it may be used as a marker for macrophage stimulation or activation.