To investigate the role of apoptosis in radiation—induced damage and repair of mouse spleen and provide the basis for expounding the corresponding mechanism as well as for the prevention and treatment of acute radiation. Using in situ terminal labeling (ISTL) , DNA electrophoresis and immunohistochemical techniques, we studied the dynamic changes of apoptosis of mouse spleen lymphocytes and the expression of bax and bcl—2 gene products after 2,4, 6 and 8Gy whole body ,γ,—irradiation. Results (1) At the early stage after irradiati on the percentages of lymphocyte apoptosis increased rapidly, for instance, 4h and day 1 after 6Gy ,γ,—irradiation the apoptotic lymphocyte were 4.9 and 9.7 times those of control values; At the same time, with the increase of radiation doses the percentages of lymphocyte apoptosis increased rapidly, for instance, 8h after 2, 4, 6 and 8 Gy ,γ,—irradiation the apoptotic lymphocyte were 3.7,5.4, 6.2 and 8. 3 times those of control values respectively. (2) After 6 Gy ,γ,—irradiation typical morphological characteristics of spleen apoptotic lymphocyte in early, middle and late stages were observed by transmission electron microscope. Using DNA gel electrophoresis technique the spleen lymphocytes displayed characteristic DNA ladders 4 and 6 h after 6 Gy ,γ,—irradiation. The percentage of lymphocyte apoptosis measured by ISTL was 3.4 times higher than that of control value 4 h after 6 Gy γ—irradiation. (3) Abnormal expression of bcl —2 and bax gene products in irradiated lymphocytes confirmed that they played a vital role in the apoptosis of mouse spleen lymphocytes. Conclusion Apoptosis played an important role in the whole process of radiation—induced damage and repair of mouse spleen lymphocyte and Bcl—2 and Bax proteins could regulate the process of lymphocyte apoptosis.