Peripheral blood mononuclear cells from normal persons were irradiated by 1—6 Gy γ-rays in vitro. NK activity was determined by means of ,3,H-TdR releasing and CD,57, positive cells were counted by means of immunocytochemistry, respectively. The results showed that NK cell activity changed unobviously whthin 2 Gy irradiation dose and dropped gradually with increasing irradiation dose and was distinguished lower than that of control when irradiation dose increased to 6 Gy. NK cell activity changed with time and was lowest at 12 hours after irradiation, then it restored gradually. The restoring level of NK cell activity was related to irradiation dose. The number of NK cell had little change till 6 Gy irradiation. The activity of NK cell increased significantly when irradiated cells were stimulated with IL-2.