Chelidonium majus is used to treat several inflammatory diseases and tumours. We have examined the effect of C. majus on nitric oxide (NO) production using mouse peritoneal macrophages. When C. majus was used in combination with recombinant interferon-gamma (rIFN-gamma, 10 U mL(-1)), there was a marked cooperative induction of NO production. Treatment of rIFN-gamma plus C. majus (1 mgmL(-1)) in macrophages caused a significant increase in tumour necrosis factor-alpha (TNF-alpha) production. The increased production of NO and TNF-alpha from rIFN-gamma plus C. majus-stimulated cells was almost completely inhibited by nuclear factor-kappaB (NF-kappaB) inhibitor, pyrrolidine dithiocarbamate (100 microM). These findings demonstrated that C. majus increased the production of NO and TNF-alpha by rIFN-gamma-primed macrophages and suggested that NF-kappaB played a critical role in mediating the effects of C. majus.