Some natural products are able to inhibit radiation effects and exert an antitumor effect withfewer adverse reactions; however, their antitumor effects are less than those of widely-usedsynthetic drugs. Propolis is a natural material that has been attracting attention, and we extractedthis material with water and investigated the effect of continuous propolis administration onradioactivity-induced reduction of hemocytes, in addition to the antioxidant and antitumoreffects of propolis. Following a 1-week adjustment period, water-soluble propolis wasadministered intraperitoneally to male ICR mice at a dose of 100 mg/kg every other day for 2weeks. Following administration, 2 Gy whole-body irradiation was performed and the counts ofleukocytes, lymphocytes, and granulocytes and monocytes in the peripheral blood weredetermined 1, 3, 7, 15 and 30 days after irradiation. These cells were considered since they areclosely associated with immunity to radioactivity. In a second experiment, water-soluble propoliswas similarly administered to the mice for 2 weeks after a 1-week adjustment period, and 2 Gywhole-body irradiation was performed. The antioxidant effects in hemocytes were theninvestigated 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, water-soluble propolis-treated, 6 Gy irradiated andwater-soluble propolis-treated+6 Gy irradiated groups, and changes in tumor size were measuredfor 20 days. Statistical analysis was conducted using ANOVA for multiple groups. In the threeexperiments, administration of water-soluble propolis inhibited the reduction of hemocytescaused by whole-body irradiation, showed antioxidant effects against radioactivity, and inhibitedtumor growth, respectively. In conclusion, our data suggest that the antioxidant effect of water-soluble propolis inhibits hemocyte reduction caused by whole-body irradiation and enhancesimmunological inhibition of tumor growth.