The radioprotective effects of a single administration of kojic acid (KA) against ionizing radiation were evaluated via assessment of 30-day survival and alterations of peripheral blood parameters of adult C57BL/6 male mice. The 30-day survival rate of mice pretreated with KA (75 or 300 mg/kg body weight, KA75 or KA300) subcutaneously 27 h prior to a lethal dose (8 Gy, 153.52 cGy/min) of gamma irradiation was higher than that of mice irradiated alone (40% or 60% vs 0%). It was observed that the white blood cell (WBC) count/the red blood cell (RBC) count, haemoglobin content, haematocrit and platelet count of mice with or without KA pretreatment as exposed to a sub-lethal dose (4 Gy, 148.14 cGy/min) of gamma irradiation decreased maximally at day 4/day 8 post-irradiation. Although the initial WBC values were low in KA300 or WR-2721 (amifostine) groups, they significantly recovered to normal at day 19, whereas in the control group they did not. The results from the cytotoxicity and cell viability assays demonstrated that KA could highly protect Chinese hamster ovary (CHO) cells against ionizing radiation with low toxicity. In summary, KA provides marked radioprotective effects both in vivo and in vitro.