The features and function of IgM-FcR of rat peritoneal macrophages were studied. Macrophages specifically bind and phagocytose ox red blood cells coated with rat IgM (EA-IgM) through a specific receptor. This receptor is trypsin sensitive and its activity requires Ca++ ions. Both sodium azide and low temperature (4 degrees) inhibit the bindings as well as ingestion of EA-IgM by macrophages, suggesting the metabolically dependent character of the interaction between EA-IgM and macrophages. Colchicine inhibits the binding of EA-IgM by macrophages. Similarly, the ingestion of EA-IgM was also inhibited when peritoneal exudate cells (PEC) were pre-treated with colchicine or vinblastine or cytochalasin B. It is suggested that cytoskeletal elements of macrophages play an important role both in the binding of EA-IgM to their receptors and in the subsequent internalization of the receptor-ligand complexes. Ingestion of soluble IgM antibodies containing immune complexes (IC) resulted in a release of beta-glucuronidase from macrophages.