Delta 9-tetrahydrocannabinol (delta 9-THC), the major psychoactive component of marijuana, has been shown to suppress macrophage soluble cytolytic activity. The purpose of this study was to determine whether delta 9-THC inhibited this function by affecting tumor necrosis factor-alpha (TNF-alpha). The RAW264.7 macrophage cell line was used as an in vitro bacterial lipopolysaccharide-inducible system for production of TNF-alpha. Macrophage-conditioned medium of RAW264.7 macrophages treated with delta 9-THC was shown to be deficient in tumoricidal activity. Immunoprecipitation experiments demonstrated that the macrophage-conditioned medium of cultures treated with drug contained lower levels of TNF-alpha. Northern analysis indicated that delta 9-THC had no effect on the levels of TNF-alpha messenger RNA. However, radiolabel pulsing and pulse-chase experiments revealed that the intracellular conversion of the 26-kD presecreted form of TNF-alpha to the 17-kD secreted form was inhibited by the drug. These results indicate that delta 9-THC suppresses soluble macrophage tumoricidal activity, at least in part, by decreasing the intracellular conversion of presecretory TNF-alpha to its 17-kD secretory form.