We investigated antitumor effects of water extracts of Cordyceps sinensis (WECS). WECS (100 microg/ml) induced apoptosis of B16 melanoma cells after 48 h exposure in vitro as determined by both the TUNEL (TdT-mediated dUTP-biotin nick end labeling) method and the detection of a DNA ladder. In vivo, combined treatment with WECS and methotrexate (MTX) in mice intravenously inoculated with B16 melanoma cells was conducted. Although MTX caused a significant and severe decrease in body weight compared with that in control mice starting 16 days after the start of administration, the mice given both MTX and WECS did not show a significant decrease in body weight. The mean survival time (days) of the control mice, MTX-treated mice (15 mg/kg/day, s.c.), and WECS-treated mice (200 mg/kg/day, p.o.) was 25.0 +/- 0.3, 25.6 +/- 1.3, and 25.7 +/- 1.0 (mean +/- S.E.M. of 6-7 mice), respectively. On the other hand, mean survival time (days) of mice given the combination of MTX and WECS was 28.2 +/- 0.7, significantly longer than the control value. WECS might be beneficial in the prevention of tumor metastasis as an adjuvant agent in cancer chemotherapy.