The anti-metastatic activity of water extract of Samguikoeuitang (WSGKE) consisting of Ginseng Radix, Angelicae Radix, Sophora flavescens and Coicis Semenwas examined. Ethanol extract of Samguikoeuitang (ESGKE) showed significant cytotoxicity against colon 26-M3.1 carcinoma cells, while WSGEK did not. However, WSGKE significantly increased the production of IL-6 and IL-12 in thioglycollate-induced macrophages from Balb/c mice, whereas ESGKE did not. WSGKE significantly increased natural killer (NK) cell cytotoxicity against effecter YAC-1 cells in an Effecter cells/Target ratio dependent manner. Also, WSGKE significantly suppressed lung metastasis after i.v. injection of colon26-M3.1 carcinoma cells. Inhibitory effect of WSGKE on lung metastasis totally abolished in NK cells-deficient mice by treatment with anti-asialo GM1 serum. In addition, the combination treatment of cisplatin and WSGKE (100 ${mu}g$/mouse) prolonged the lifespan of mice inoculated by colon26-M3.1 cell. These findings suggest that WSGKE can inhibit lung metastasis and prolong life span via immunological enhancement as a Biological Response Modifier.