[en] The author investigated the effects of a radioprotective agent (YM 08310 = WR-2721) on osteoradionecrosis of the mandible. External X ray irradiation was performed on right side of the mandible. The dosage of irradiation was 3000 R. The rabbits were administrated of YM 08310 100 mg/kg or 200 mg/kg intra-venously 30 min. before irradiation. The results were the following. 1. Group without YM 08310 (only irradiation): One month after irradiation, there was almost no resorption of bone observed. 3 months after irradiation, most of the cortical bone was resorbed and was replaced by fibrous connective tissue. The formation of new bone by endosteum also was observed. 6 months after irradiation, some dead bone was replaced by new bone, but in the cortical bone there was still bone resorption observed. 2. Group with YM 08310 100 mg/kg: One month after irradiation, there was almost no bone resorption observed. 3 months after irradiation, there was resorption in cortical bone, and the formation of new bone by the periosteum and the endosteum observed. 6 months after irradiation, there was still a little resorption in the cortical bone but vigorous bone formation was observed. 3. Group with YM 08310 200 mg/kg: One month after irradiation, there was almost no bone resorption observed. 3 months after irradiation, there was no dead bone and no remarkable resorption observed and the same findings were observed for 6 months after irradiation. From these results, the author drew the following conclusion. 1. There were effects of YM 08310 on osteoradionecrosis observed, and the effects were more prominent in the group with 200 mg/kg than in that with 100 mg/kg. 2. In the group with YM 08310 100 mg/kg, there were still bone addition and repair by periosteum after irradiation, and that was due to the effect of YM 08310 on periosteum. 3. In the group with YM 08310 200 mg/kg, there was a remarkable effect of YM 08310 on the bone and this agent could prevent osteoradionecrosis. (author)