Lipopolysaccharide (LPS) was shown to induce tumoricidal activity in peritoneal macrophages. The optimal concentration was found to be 25 micrograms/mL. Approximately 20-h exposure to LPS was required before maximal tumor cell killing was attained. Optimal tumor killing was obtained with a ratio of tumor cell to macrophages of 1:40 with the macrophages in a confluent layer. Diphenylene iodonium (DPI) reduced the tumor cell killing in a dose dependent manner up to 1 microM. Under similar conditions DPI was shown to inhibit the superoxide production of macrophages and this supports the view that the production of oxygen radicals is important in the killing of tumor cells by macrophages and that the inhibitor DPI can be used to investigate their contribution to cytotoxicity.