Mouse spleen cell-myeloma hybrids were produced that secreted monoclonal antibodies to SRBC of the IgG2a and IgG2b subclass. These homogeneous antibodies were used to examine the Fc receptors for mouse IgG2a and IgG2b on resident and thioglycollate-induced macrophages and on two continuous macrophage-like cell lines (J774.2 and FC-1). By cross-inhibition experiments, we have shown that all four of these cell types have separate receptors for aggregated IgG2a and IgG2b. Both receptors bind heterogeneous rabbit antibody complexed to antigen. Large amounts of soluble proteins do not inhibit the binding of antigen-antibody complexes to either receptor. The rosetting of IgG2a-coated SRBC, but not of IgG2b-coated SRBC, is inhibited by treating all four cell types with trypsin or cytochalasin and is decreased at 4°C. Both the IgG2a and IgG2b receptors will mediate phagocytosis in all four cell types. Mutants of the FC-1 cell line were obtained that can bind both IgG2a and IgG2b antibody-antigen complexes but phagocytize only through the IgG2b receptor. We conclude from these studies that naturally occurring complexes between antigen and mouse IgG2a and IgG2b antibodies are bound to mouse macrophages and phagocytized through functionally distinguishable Fc receptors. The use of homogeneous monoclonal antibodies facilitates such studies and avoids some of the ambiguities that arise when artificially aggregated immunoglobulins or heterogeneous antibodies are used.