The EMT6 mouse tumor was selected for use in the study of cytocidal activity and proliferative ability of infiltrating macrophages because of its high plating efficiency when explanted to culture. The plating efficiency for cells directly plated in culture from the tumor was 28 +/- 9.3%, irrespective of the size of the tumor. Of the adherent cells derived from the tumor, the fraction that was macrophages increased from 27% at 7 days to 47% at 28 days after initial injection. Time-lapse cinemicrography was used to directly observe adherent cells derived from the tumor, and macrophages were found to be cytocidal. When grown in the presence of L-cell conditioned medium no macrophage colonies were found when cultures were established from untreated mice even though most tumor cell colonies contained macrophages. When mice were first treated with 0.8 mg BCNU prior to establishment of the cultures, in order to reduce the frequency of colony-forming tumor cells, approximately half the colonies found contained only macrophages. These results show that macrophages in this tumor are cytocidal and capable of proliferation.