In studies on the functional activity of macrophages isolated from murine sarcoma virus (MSV)-induced tumors, we have found that these cells may suppress immune responses as well as act as effector cells against the tumor. Previously, we reported that macrophages from the tumor could inhibit the antitumor response by suppressing proliferation-dependent immune functions. Here, we demonstrate that macrophages can also suppress the production of migration inhibition factor (MIF) and macrophage activation factor (MAF), two lymphocyte activities that are independent of cell proliferation. Conversely, we and others have found that macrophages from the tumor can exert an antitumor, cytolytic effect. In this study, using 1 g velocity sedimentation separation techniques, we have been able to identify 2-3 subpopulations of cytolytic macrophages in regressing tumors but in progressing tumors, only the smallest subpopulation of macrophages was active. T cells appeared to be required for activation of macrophages within the tumor, since MSV tumors induced in athymic, nude mice did not contain cytolytic macrophages.