Cytotoxic macrophages have been isolated from Moloney sarcoma virus (MSV) tumors induced in uncompromised, immunosuppressed and athymic nude mice. Macrophages from compromised mice were at least as active as those from uncompromised mice when tested against the autochthonous MSV target cells. Although both cytolytic and cytostatic activity could be demonstrated with all tumor-derived macrophages, cytostatic and cytolytic effects could be distinguished from each other only at high effector target-cell ratios. A variety of target cells were compared for sensitivity to the various tumor-associated macrophages. The autochthonous virus-infected MSV-target cells appeared to be more sensitive to the macrophage effects than fastgrowing, established cell lines. Normal embryo fibroblasts were only slightly affected. Peritoneal exudate cells from tumor-bearers were much less active than either the tumor-isolated macrophages or bone-marrow culture-derived macrophages. Both were more active than control peritoneal cells. Although no quantitative or qualitative differences in macrophage activity could be detected in mice with regressing lesions and immuno-compromised mice with progressing lesions, the ratio of macrophages to "sarcoma" cells was three to five times higher in regressing tumors.