Using mouse peritoneal macrophages, we have examined the mechanism by which Ixeris dentata (IXD) regulates nitric oxide (NO) production. When IXD was used in combination with recombinant interferon-gamma (rIFN-gamma), there was a marked cooperative induction of NO production. However, IXD had no effect on NO production by itself. The increased production of NO from rIFN-gamma plus IXD-stimulated cells was almost completely inhibited by pre-treatment with pyrrolidine dithiocarbamate (PDTC), an inhibitor of nuclear factor kappa B (NF-kappaB). Furthermore, treatment with IXD alone or rIFN-gamma plus IXD in peritoneal macrophages caused a significant increase in tumor necrosis factor-alpha (TNF-alpha) production. PDTC decreased TNF-alpha production induced by IXD significantly. These findings demonstrate that IXD increases the production of NO and TNF-alpha by rIFN-gamma-primed macrophages and suggest that NF-kappaB plays a critical role in mediating these effects of IXD.