Delta 9tetrahydrocannabinol (THC), the major psychoactive component of marijuana, has been shown to inhibit macrophage cell contact-dependent cytolysis of tumor cells. The purpose of this study was to determine whether THC inhibited macrophage cytolytic function by targeting selectively tumor necrosis factor (TNF)-dependent pathways versus L-arginine-dependent reactive nitrogen intermediates. An in vitro system employing RAW264.7 macrophage-like cells as effectors and TNF-sensitive mouse L929 fibroblasts or nitric oxide (NO.)-sensitive P815 mastocytoma cells as targets, was employed to assess the effect of THC on cytolysis. Macrophages were pretreated with THC or vehicle for 48 hr, subjected to multistep activation with 10 U/ml recombinant mouse gamma-interferon (IFN-gamma) plus 100 ng/ml LPS or to direct activation with 1 microgram/ml LPS, and co-cultured with tumor cells in the presence or absence of THC. THC inhibited TNF-dependent killing by macrophages subjected to either multistep or direct activation. Decreased amounts of TNF-alpha were detected in medium of macrophage cultures treated with THC. In contrast, THC inhibited NO.-dependent cell contact killing only for macrophages subjected to direct activation. Decreased levels of NO2-, a stable degradation product of the short-lived and highly toxic effector molecule NO., were produced by these macrophages. In addition, the effect of the enantiomeric pairs (-)CP55,940/(+)CP56,667 or (-)HU-210/(+)HU-211 on macrophage cell contact-dependent killing was assessed. Inhibition of macrophage tumoricidal activity against TNF-sensitive L929 cells was effected by both isomers of THC analogs. In contrast, both of the enantiomeric pairs had an effect on killing of NO.-sensitive P815 mastocytoma cells only for macrophages subjected to direct activation. These data suggest that cannabinoids inhibit macrophage cell contact-dependent killing of tumor cells by a noncannabinoid receptor-mediated mechanism. However, specific cytolytic pathways are inhibited differentially by cannabinoids depending on the activation stimuli to which macrophages are exposed.