Although the protective effect of the polyhydroxylated fullerene derivative C(60)(OH)(n) against ionizing radiation is an area of much interest, the mechanisms relating to how polyhydroxylated fullerene derivatives improve mitochondrial dysfunction remain unknown. In order to find new and effective radioprotective agents, we synthesized a new polyhydroxylated fullerene molecule with 24 hydroxyl groups of known positions on C(60) and studied its protective effects in mice subjected to irradiation. Mice were pretreated with C(60)(OH)(24) for 2 weeks (daily, 40 mg/kg i. p.), then subjected to a lethal dose of whole body gamma-irradiation (from a (60)Co source). Survival was observed for 30 days after irradiation. Immune and mitochondrial dysfunction and oxidative damage were analyzed in mice with the same C(60)(OH)(24) pretreatment and irradiation except that the animals were euthanized at day 5 after the irradiation. It was found that 2-week C(60)(OH)(24) pretreatment effectively reduced whole body irradiation-induced mortality without apparent toxicity. C(60)(OH)(24) pretreatment also showed significant protective effects against ionizing-radiation-induced decreases in immune and mitochondrial function and antioxidant defense in the liver and spleen. These results suggest that the polyhydroxylated fullerene derivative C(60)(OH)(24) protects against ionizing-radiation-induced mortality, possibly by enhancing immune function, decreasing oxidative damage and improving mitochondrial function.