By using TdT-mediated dUTP nick end labeling (TUNEL),in situ, hybridization and immunohistochemical method of alkaline phosphatase, we observed the apoptotic dynamic changes of mouse lymph node lymphocyte after lethal dose γ-ray irradiation, and its relationship to the expressions of Bax, Bcl-2 and Bcl-X,L, proteins and bax, bcl-2 mRNAs.The results showed that 24h after irradiation, apoptotic index of lymph node lymphocytes increased swiftly, with a positive correlation to the doses within 6—12Gy, whereas no relationship was observed for 15Gy and 20Gy irradiation groups. Twenty-four hours after 6Gy irradiation, the lymphocytes apoptosis reached a maximal level and decreased thereafter. However, up to 6 and 12 months after the 6Gy irradiation, the apoptotic index was still higher than the control group. Twenty-four hours after the irradiation, the expression of lymphocyte Bax protein enhanced immediately, with the 12Gy group being highest value, whereas such a dose-effect was not found for the 15Gy and 20Gy groups. While the expressions of Bcl-2 and Bcl-X,L, proteins decreased evidently 24h after the irradiation, the analysis on bax and bcl-2 mRNA showed also a similar tendency. It was suggested that apoptosis was an important way of lymphocyte death pathways after ≤12Gy irradiation. The up-regulation of Bax and down-regulation of Bcl-2 or Bcl-X,L, indicate that they play an important role in the apoptotic regulation of lymphocytes induced by lethal dose radiation.