The protective effect of 5,6,7,8-tetrahydroneopterin (NH.) against radiation injury in mice was studied. (C57BL/6x A/J)F mice received whole body irradiation with a single fatal dose of 8.0 Gy X-ray. NH was injected intraperitoneally into irradiated mice at various protocols. NH. was significantly effective to prolong the survival of the irradiated mice. The most effective protocol of NH administration was 3 times injections (10 minutes before and after irradiation and additional one 12 hours after that.) with a dose of 30 mg/kg body weight for each. Some mice that received such treatment survived more than 30 days after irradiation. In conclusion, these results indicate that NH has an inhibitory potential on radiation injury as an endogenous superoxide dismutase like substance.