The dose_effect of adaptive response of thymocyte apoptosis and cell cycle progression induced by whole_body X_ray irradiation (WBI) was studied in male Kunming mice. The inductive doses (D-1) were 25, 50, 75, 100 or 200 mGy 6 h before the challenging doses (D-2) of 1.0, 1.5 or 2.0 Gy. The changes in the percentages of the thymocyte apoptotic bodies (TAB) and the cells in different phases of cell cycle were measured with flow cytometry. The percentages of TAB decreased, the arrests of G-1 and G-2+M phases diminished, and the cells of DNA synthesis of S phase increased when the D-1+D-2 groups was compared with the D-2 groups. When D-1 was 200 mGy, the adaptive response of thymocyte apoptosis and cell cycle progression were no longer induced by low dose radiation (LDR). In addition, the extracellular fluid from the splenocytes were cultured with Con A for 48 h %in vitro% 24 h after 75 mGy WBI was placed in the murine thymocyte suspension from mice irradiated with 2.0 Gy WBI and co_incubated. The thymocyte apoptosis decreased. Especially, noteworthy was that the percentages of TAB after the incubation for 72 h were significantly lower than those in 2.0 Gy irradiated thymocytes (%P%<0.05). These results indicate that when the mice were irradiated with 25_100 mGy (D-1, 12.5 mGy/min) 6 h before 1.0_2.0 Gy (D-2, 0.287 Gy/min) exposure, an adaptive response of thymocyte apoptosis and cell cycle progression may be induced under the condition of WBI, and LDR (75 mGy) may change the microenvironment of immune cells and decrease the thymocyte apoptosis. The dose_effect of adaptive response of thymocyte apoptosis and cell cycle progression induced by whole_body X_ray irradiation (WBI) was studied in male Kunming mice. The inductive doses (D - 1) were 25, 50, 75, 100 or 200 mGy 6 h before the challenging doses (D -2) of 1.0, 1.5 or 2.0 Gy. The changes in the percentages of the thymocyte apoptotic bodies (TAB) and the cells in different phases of cell cycle were measured with flow cytometry. The percentages of TAB decreased, the arrests of G -1 and G -2 + M phases diminished, and the cells of DNA synthesis of S phase increased when the D - 1 + D - 2 groups was compared with the When D - 1 was 200 mGy, the adaptive response of thymocyte apoptosis and cell cycle progression was no longer induced by low dose radiation (LDR). In addition, the extracellular fluid from the splenocytes were cultured with Con A for 48 h % in vitro % 24 h after 75 mGy WBI was placed in the murine thymocyte suspension from mice irradiated with 2.0 Especially, noteworthy was that the percentages of TAB after the incubation for 72 h were significantly lower than those in 2.0 Gy irradiated thymocytes. (% P % <0.05). These results indicate that these percentages of TAB after the incubation for 72 h were significantly lower than those in 2.0 Gy irradiated thymocytes that when the mice were irradiated with 25_100 mGy (D -1, 12.5 mGy / min) 6 h before 1.0_2.0 Gy (D -2, 0.287 Gy / min) exposure, an adaptive response of thymocyte apoptosis and cell cycle progression may be induced under the condition of WBI, and LDR (75 mGy) may change the microenvironment of immune cells and decrease the thymocyte apoptosis.