The ability of several immunologic ligands to mediate extracellular cytolysis of sheep erythrocytes (E) by mouse macrophages was studied. E coated with rabbit IgG (EIgG), but not nonsensitized E or E coated with rabbit IgM (EIgM), were lysed and phagocytized by resident peritoneal macrophages as well as by macrophages activated in vivo with either BCG or Corynebacterium parvum or in vitro with lymphokine. EIgM incubated with mouse serum to deposit the third component of complement (C3) onto E (EIgMC) were ingested by thioglycolate-elicited and lymphokine-treated macrophages, but not by the other macrophage populations examined. However, none of the macrophages performed lysis of EIgMC, suggesting that engagement of macrophage C3 receptors by target cell-bound C3 was not a sufficient trigger for cytolysis. Lysis of E coated with both IgG and C3 (EIgGC) was moderately elevated over that of EIgG; this enhancement was not abolished after proteolytic destruction of the macrophage C3-rosetting capacity, indicating that C3 receptors were not responsible. EIgGC and EIgMC were more susceptible to hypotonic lysis than were either E, EIgG, or EIgM, suggesting that enhanced lysis of serum-treated E may be partially explained by increased E fragility.