Antimacrophage sera (AMS) were prepared in rabbits by injection of mouse peritoneal cells with or without purification by in vitro culturing. These AMS showed high cytotoxic activity against macrophages in culture. The antimacrophage sera were not specific for macrophages; they cross-reacted and were cytotoxic to lymphocytes and granulocytes in the in vitro assay. Injection of AMS into mice did not affect antiShigella antibody production, nor did it prolong skin allograft survival. Furthermore, AMS-treated macrophages did not lose their ability to induce antiShigella antibody production in irradiated mice. On the other hand, AMS inhibited phagocytosis of Bacillus subtilis by macrophages and likewise increased the synthesis of DNA in cultured macrophages.