Various in vitro studies have shown that delta-9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, has a variety of inhibitory effects on immune functions including effects on macrophages. The present studies have examined the mechanism of THC's effects on tumor necrosis factor alpha (TNF-alpha), a major macrophage-produced cytokine and an important mediator involved in cytokine networks and in host defense mechanisms. Exposure of macrophages to medium containing THC has resulted in low levels of soluble TNF-alpha protein and reduced TNF-alpha bioactivity in the culture supernatant. However, THC did not inhibit the levels of LPS-induced TNF-alpha mRNA and intracellular TNF-alpha precursor protein, had only a weak effect on expression of membrane-bound TNF-alpha, but suppressed TNF-alpha maturation/secretion by macrophages. The higher the THC concentration in the medium during TNF-alpha induction, the greater the amount of intracellular TNF-alpha precursors that accumulated in the activated macrophages and the less mature TNF-alpha was released from the cells. Data suggest that TNF-alpha production by macrophages was altered greatly by exposure to THC at the levels of TNF-alpha precursor maturation and secretion.