Purpose :. To investigate the effect of aqueous extract of Ganoderma lucidum(G.I.) on the surival of tumor cells in vitro and on the growth of tumors in vivo. Materials and Methods : Dried G.I. was made into powder, extracted with distilled water, filtered and diluted from a maximum concentration of 100 mg/ml in sequence. The cytotoxicity of G.I, in vitro was evaluated from its ability to reduce the clonogenicity of SCK tumor cells. For the tumor growth delay study, about $2{ imes}10^5$$ of SCK tumor cells were subcutaneously inoculated in the legs of A/J mice. The first experimental group of mice were injected i.p. with 0.2ml of 250 mg/kg of G.I. from the first day after tomor inoculation for 10 days. The second experimental group of mice were injected i.p. with 0.2ml of 250 mg/kg of G.I. either once a day for 10 days or twice a day for 5 days beginining from the 7th day after tumor inoculation Results : 1. Cytotoxicity in vitro;survival fraction, as judged from the curve, at G.I. concentration of 0.5, 1,5, 10, 25, 50 and 100 mg/ml were 1.0, $0.74{pm}0.03$, $0.18{pm}0.03$, $0.15{pm}0.02$, $0.006{pm}0.002$, 0.015 and 0.0015, respectively. 2. Tumor growth delay in vivo; a) the time required for the mean tumor volume to grow to $1,000mm^3$ was 11 days in the control group and 14 days in the experimental group. b) the time required for tumor volume to increase 4 times was 11 days in the control group while it was 10.5 and 12 days in the groups injected with G.I. once a day and twice a day from the 7th day after tumor inoculation respectively. Conclusion : Aqueous extracts of G.I. showed a marked cytotoxicity on the SCK mammary cells in vitro. Tumor growth delay was statistically signiricant when G.I. in-jection was started soon after tumor inoculation, but it was not significant when injection was started after the tumors were firmly established.