Nitric oxide, a proposed mediator of macrophage tumour cell killing, can react with superoxide to form peroxynitrite, which is capable of reacting with a variety of biological molecules. The RAW 264 murine macrophage cell line can be stimulated by cytokines and lipopolysaccharide to produce NO by induction of the inducible nitric oxide synthase. We have shown that activated RAW 264 cells are cytotoxic towards P815 murine mastocytoma cells, and that addition of a nitric oxide synthase inhibitor exerts a concentration dependent protective effect. Addition of superoxide dismutase had no effect on either RAW 264 NO production or cell killing. It was shown that RAW 264 cells do not undergo an oxidative burst. These results indicate that RAW 264 tumour cell killing is primarily a nitric oxide mediated event and does not involve macrophage-derived superoxide.