Peripheral blood of healthy volunteers was irradiated with ⁶⁰Co in order to study the effects of radiation on superoxide anion radical (O₂^-_·) generation and the inhibitory effects of α₂-macroglobulin (α₂M) blood preparation. It was found that one hour after irradiation, the amount of O₂^-_· released from polymorphonuclear leukocytes (PMNs) was more than that of the non-irradiated group (P＜0.01) and the activities of superoxide dismutase (SOD) in red cells decreased (P＜0.01). When α₂M (138.5 U/ml blood) were added one hour before irradiation, the amount of O₂^-_· released from PMNs were reduced and the activities of SOD in red cells were preserved. These results suggest that one reason of α₂M against radiation may be related to inhibition of O₂^-_· released from PMNs.