Macrophages isolated from regressing or progressing tumors induced by murine sarcoma virus (MSV) were tested for cytolytic activity in a 18-h 51Cr release assay. Macrophages from the tumors of mice injected 14 days earlier with stocks of MSV-producing regressor or progressor tumors had comparable levels of cytotoxicity. However, macrophages from the progressively growing tumors, 50--65 days after the inoculation with the progressor virus, had lower levels of cytotoxicity than those from the regressing tumors (day 14). On the other hand, macrophages from progressively growing MSV tumors (day 14) in nude mice had little or no detectable cytolytic activity. In a fractionation study, using the 1-g velocity sedimentation technique, cells from regressing tumors (day 14) showed at least two peaks of cytolytic activity, one at about 4 mm/h and another at 6--7 mm/h. In contrast, none of the cell fractions of tumors from nude mice (day 14) showed cytolytic activity. Since bacterial lipopolysaccharide (LPS) has been shown to augment the cytolytic activity of primed macrophages, its effect on cells from MSV tumors was evaluated. Cytolytic activity of the cells from regressing or progr-ssing tumors that had pre-existing cytolytic activity was augmented by the addition of LPS during the assay period, but LPS treatment did not activate inactive fractions or change the distribution patterns of the cytolytic activity in fractions from 1-g velocity sedimentation.