1. The synthesis of nitric oxide (NO) by immune-stimulated murine phagocytic cells (J774) and the modulation of this synthesis by tricyclodecan-9-yl-xanthogenate (D609), a specific inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), was investigated. D609 dose-dependently suppressed production of NO, as measured by the release of nitrite and nitrate, in response to lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) in intact cultured cells with an IC50 of approximately 20 micrograms ml-1. D609 at 40 micrograms ml-1 completely abrogated immune-stimulated nitrite production. 2. The inhibitory effects of D609 on nitrite production were time-dependent and restricted to the first 18 h post-stimulation. D609 did not inhibit nitrite production in the cytosol of immune-stimulated phagocytes. 3. These findings indicate that the xanthogenate, D609, is a potent inhibitor of the induction of NO-synthase activity in immune-stimulated phagocytes. Furthermore, since D609 has been demonstrated to inhibit PC-PLC specifically, our findings suggest that the activation of this enzyme by LPS and IFN-gamma is a proximal step in the signal transduction of inducible NO-synthase in phagocytic cells.