Cells of the mononuclear phagocyte system can be activated by lymphokines to both increased extracellular cytotoxicity against tumor targets and intracellular cytotoyicity against micro-organisms. In addition, these effector cells can kill antibody-coated target cells in an ADCC system. These two cytotoxic mechanisms can cooperate and act synergistically. Such an synergistic action is characterized by the, specificity of the antibody which coats the target cells and not by the non-specific activation induced by high dosages of lymphokine. The lymphokine MCF has partially been purified and separated from a variety of other lymphokines. This purified material, when injected into mice intraperitoneally, activates the macrophages to strong cytotoxicity. Evidence obtained by the use of different rat anti-mouse macrophage monoclonal antibodies suggests that there exist different subpopulations of macrophage and that some of these subpopulations can be correlated to defined functions.