PURPOSE: To investigate the effect of aqueous extract of Ganoderma lucidum(G.I.) on the survival 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.O. in vitro was evaluated from its ability to reduce the clonogenicity of SCK tumor cells. For the tumor growth delay study, about 2' 105 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 tumor 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 beginning 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+/-0.03, 0.18+/-0.03, 0.15+/-0.02, 0.006+/-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 1,000mm3 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 significant when G.I. injection was started soon after tumor inoculation, but it was not significant when injection was started after the tumors were firmly established.