In this experiment, we show the effects of herbal plant extracts on the production of nitric oxide (NO) and TNF-$alpha$. The extracts of Angelica gigas, Astragalus membranaceus, Acanthopanax sessiliflorus and Houttuynia cordata had no effect on NO synthesis by itself in mouse macrophage cell line (RAW264.7). However, the stimulation with these extracts in the presence of murine interferon-${gamma}$(mIFN-${gamma}$) resulted in increased NO synthesis. When these extracts were used in combination with mIFN-${gamma}$, there were a marked cooperative induction of NO and TNF-$alpha$ synthesis in a dose-dependent manner. The same results were obtained in the mouse peritoneal macrophages used. The optimal concentration of these extracts on NO synthesis was shown at 100$mu extrm{g}$/mL with 100U/mL of mIFN-${gamma}$. NO synthesis was inhibited by NG-monomethyl-L-arginine. When cell lines were treated with extracts, the expression of inducible NO synthetase (iNOS) was markedly increased in RT-PCR analysis. In addition, synergy between mIFN-${gamma}$ and extracts was dependent on extracts-induced tumor necrosis factor-$alpha$(TNF-$alpha$). These results suggest that water extracts of herbal plants can induce iNOS, NO and TNF-$alpha$ synthesis of mouse macrophage cell line (RAW264.7) and peritoneal macrophages in combination with mIFN-${gamma}$.