The present study was designed to investigate the effects of Stachys Sieboldii MIQ as a new natural antitumor agent or immunomodulator. To obtain the above objectives, Stachys sieboldii MIQ was extracted with ethanol. Stachys sieboldii MIQ accelerated mouse spleen cell growth, but inhibited FM3A/S°-cell growth. However, no significant difference was found for CD4+ / CD8+ cells. The growth rates of CD4+ and CD8+ T cells were accelerated more than those normal mouse group. Stachys sieboldii MIQ fed mice showed a significant enhancement of IL-2 receptor expression, increased numbers of CD4+ T cells, and CD8+ T cells. Stachys sieboldii MIQ also stimulated the production of NO by peritoneal macrophages and the production of NO by and the growth of mouse spleen cells. On the other hand, lung localization of Bl6Fl0 melanoma cells was inhibited by ethanol extract of Stachys sieboldii MIQ. These results show that Stachys sieboldii MIQ is a useful new functional antitumor agent or immunomodulator.