The ability of human mononuclear phagocytic cells to bind IgG-coated human erythrocytes (EA) and to cause bound EA to become osmotically fragile (sphered) was investigated in the presence of cytochalasin B, a known inhibitor of phagocytosis. Cytochalasin B inhibited the binding of EA to mononuclear cells in a dose-dependent fashion; 80% inhibition of binding was observed at a concentration of 5 mug/ml. This profound effect on EA binding together with presently available data suggested a role for IgG receptor mobility in the macrophage binding of IgG-coated erythrocytes. Cytochalasin B, however, had a minimal effect on the capacity of mononuclear cells to sphere adherent EA, suggesting that the processes involved in macrophage-induced spherocytosis may differ from those operable in phagocytosis.