Some natural products are able to inhibit radiation effects and exert an antitumor effect with fewer adverse reactions; however, their antitumor effects are less than those of widely-used synthetic drugs. AHCC (Active Hexose Correlated Compound) is a natural material that has been attracting attention, and we extracted this material with water and investigated the effect of continuous AHCC administration on radioactivity-induced reduction of hemocytes, in addition to the antioxidant and or effects of AHCC. Following a 1-week adjustment period, AHCC was administered intraperitoneally to male ICR mice at a dose of 250mg/kg every other day for 2weeks. Following administration, 2Gy whole-body irradiation was performed and the counts of leukocytes, lymphocytes, and granulocytes and monocytes in the peripheral blood were determined 1, 3, 7, 15 and 30days after irradiation. These cells were considered since they are closely associated with immunity to radioactivity. In a second experiment, AHCC was similarly administered to the mice for 2weeks after a 1-week adjustment period, and 2Gy whole-body irradiation was performed. The antioxidant effects in hemocytes were then investigated using 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH), a radical generator. In a third experiment, 1106 Sarcoma-180 cells were inoculated into the right thigh of mice, which were divided into four groups: control, AHCC-treated, 6Gy partial irradiated and AHCC-treated + 6Gy partial irradiated groups, and changes in tumor size were measured for 20days. Statistical analysis was conducted using ANOVA for multiple groups. In the three experiments, administration of AHCC inhibited the reduction of hemocytes caused by whole-body irradiation, showed antioxidant effects against radioactivity, and inhibited tumor growth, respectively. In conclusion, our data suggest that the antioxidant effect of AHCC inhibits hemocyte reduction caused by whole-body irradiation and enhances immunological inhibition of tumor growth.