We investigated the effect of the water extract of Cordyceps sinensis (WECS) on liver metastasis of Lewis lung carcinoma (LLC) and B16 melanoma (B16) cells in mice. C57BL/6 mice were given a s.c. injection of LLC and B16 cells and sacrificed 20 and 26 days after tumor inoculation, respectively. WECS was daily administered p.o. to the mice in a dose of 100 mg/kg body weight (wt.) in the experiment of LLC and in a dose of 100 or 200 mg/kg body wt. in the experiment of B16 from one week before tumor inoculation to one day before the date of sacrifice. The tumor cells increased in the thigh in LLC-inoculated mice and in the footpad in B16-inoculated mice. The relative liver wt. of the tumor-inoculated mice significantly increased as compared to that of the normal mice due to the tumor metastasis, as verified by the hematoxylin-eosin staining pathological study in the LLC experiment. The relative liver wt. of the WECS-administered mice significantly decreased relative to that of the control mice in both the LLC and B16 experiments. WECS showed a strong cytotoxicity against LLC and B16 cells, while cordycepin (3'-deoxyadenosine), an active component of WECS, was not cytotoxic against these cells. These findings suggest that WECS has an anti-metastatic activity that is probably due to components other than cordycepin.