The low dose hyper-radiosensitivity (HRS) in human lung cancer cell line A549 was in-vestigated,the changes of ATM kinase,cell cycle and apoptosis of cells at different doses of radiation were observed,and the possible mechanisms were discussed.A549 cells in logarithmic growth phase were irradiated with 60Co γ-rays at doses of 0-2 Gy.Together with flow cytometry for precise cell sorting,cell survival fraction was measured by means of conventional colony-formation assay.The expression of ATM1981Ser-P protein was examined by Western blot 1 h after radiation.Apoptosis was detected by Hoechst 33258 fluorescent staining,and Annexin V-FITC/PI staining flow cytometry 24 h after radiation.Cell cycle distribution was observed by flow cytometly 6,12 and 24 h after ra-diation.The results showed that the expression of ATM1981Ser-P protein was observed at 0.2 Gy,followed by an increase at >0.2 Gy,and reached the peak at 0.5 Gy,with little further increase as the dose exceeded 0.5 Gy.Twenty-four h after radiation,partial cells presented the characteristic mor-phological changes of apoptosis,and the cell apoptosis curve was coincident with the survival curve.As compared with control group,the cell cycle almost had no changes after exposure to 0.1 and 0.2 Gy radiation (P>0.05).After exposure to 0.3,0.4 and 0.5 Cry radiation,G2/M phase arrest occurred 6 and 12 h after radiation (P