Monoclonal antibodies to sheep erythrocytes (SRBC) have proved useful in identifying two Fc receptors on mouse macrophages, one for IgG2a, and one for IgG1 and IgG2b. We have used monoclonal IgG3 anti-SRBC to identify a third Fc receptor on mouse macrophages which binds IgG3 uniquely. This receptor is present on primary resident and thioglycolate-induced peritoneal macrophages and on some macrophage cell lines. The binding of IgG3-coated SRBC is inhibited by aggregated byt not monomeric IgG3, and not by IgG1, IgG2a, and IgG2b aggregates. It is unaffected by treating the macrophages with trypsin or cytochalasin B and occurs at both 4 degrees and 37 degrees C. IgG3, like all other IgG subclasses, mediates phagocytosis. We have also generated a variant macrophage line which bears the receptors for IgG1 and IgG2b and for IgG2a, but not for IgG3.