A warm water-extract (ECS) prepared from dried Cordyceps sinensis (Berk.) Sacc., a Chinese traditional medicine, was tested for antitumor activity in vivo and in vitro. Ehrlich ascites carcinoma cells (EAC), allogeneic to ICR mice and Meth A fibrosarcoma (Meth A), syngeneic to BALB/c mice were used as the target tumor cell lines. Mice were inoculated i.p. with 1 x 10(6) EAC or 1 x 10(5) Meth A on Day 0, and ECS or saline (control) was injected i.p. to the mice from Day 1 to Day 4. ECS-treatment increased the median survival time of the allogeneic mice inoculated with EAC to 316% of the control. Eight of the 10 ECS-treated mice survived on the 60th day (Day 60) after EAC implantation. ECS-treatment also increased the median survival time of the syngeneic mice inoculated with Meth A to 312% of the control. Half of the ECS-treated mice survived on Day 60. On the other hand, no cytotoxic effect of ECS was found on either EAC or Meth A in vitro. The antitumor effect of ECS seen in the allogeneic mice was significantly reduced when the mice received whole body X-irradiation (5 Gy) before EAC implantation. These results suggest that the antitumor effect of ECS may be mediated through its immunomodulating action.