Low dose radiation induces apoptosis and Bcl-2 can inhibit apoptosis induced by a variety of stimuli. The effects and possible mechanisms of Bcl-2 on 4 Gy X-ray-induced apoptosis in Bcl-2 transfected and untransfected cells were studied. The data present here suggested that overexpression of Bcl-2 significantly inhibited X-ray-induced apoptosis in CHO cells within 72 h. Further investigation demonstrated that Bcl-2 prevented the loss of mitochondrial membrane potential (deltapsim) induced by X-ray irradiation. It appears that overexpression of Bcl-2 blocked the ionizing radiation-induced apoptosis. The maintenance of deltapsim plays an important role in the protective mechanism of Bcl-2 on ionizing radiation-induced apoptosis.