Cytostasis and cytotoxicity as well as survival and growth of EMT6 cells following exposure to tumoricidal macrophages have been examined using multiparameter flow cytometry. Macrophages were resolved from tumor cells, and the number of surviving EMT6 cells at different interaction times was determined by adding, to each cell suspension, fluorescent latex particles the concentration of which was known and counting them along with the cells. The percentages of EMT6 cells in S phase were determined by analysis of DNA histograms and compared to the percentages as determined by autoradiography. The progression of cells through the cell cycle was also examined during exposure to macrophages by dual parameter analysis of DNA content and bromodeoxyuridine incorporated into DNA prior to exposure to macrophages. The results showed that, by 4 h of interaction with tumoricidal macrophages, EMT6 cells stopped progressing; tumor cell progression was inhibited in all phases of the cell cycle. By 24 h, there was an absolute decrease in survival. By 48 h, however, surviving tumor cells which escaped the tumoricidal activity of macrophages exhibited a population doubling rate similar to control cells.