The aim of the present study was to determine whether the sensitivity of thymocytes to X-ray radiation depends on their proliferative states and whether radiation impairs the maturation of donor-derived thymocytes in recipient thymus.We assigned 8-week-old C57BL/6J mice into three treatment groups:1) untreated;2) X-ray radiation;3) X-ray radiation plus bone marrow transplantation with donor bone marrow cells from transgenic mice express-ing enhanced green fluorescent protein(GFP) on a universal promoter.After 4 weeks,the size of the thymus,the number and proliferation of thymocytes and ratios of different stage thymocytes were analyzed by immunohisto-chemistry and flow cytometry.The results showed that:1) CD4+CD8+ thymocytes were more sensitive to X-ray radiation-induced cell death than other thymocytes;2) the proliferative capacity of CD4+CD8+ thymocytes was higher than that of other thymocytes;3) the size of the thymus,the number of thymocytes and ratios of thymo-cytes of different stages in irradiated mice recovered to the normal level of untreated mice by bone marrow trans-plantation;4) the ratio of GFP-positive CD4+CD8+ thymocytes increased significantly,whereas the ratio of GFP-positive CD4+ or CD8+ thymocytes decreased significantly.These results indicate that the degree of sensitivity of thymocytes to X-ray radiation depends on their proliferative states and radiation impairs the maturation of donor-derived CD4+CD8+ thymocytes in recipient thymus. The aim of the present study was to determine whether the sensitivity of thymocytes to X-ray radiation depends on their proliferative states and whether radiation impairs the maturation of donor-derived thymocytes in recipient thymus. We assigned 8-week-old C57BL / 6J mice into three treatment groups: 1) untreated; 2) X-ray radiation; 3) X-ray radiation plus bone marrow transplantation with donor bone marrow cells from transgenic mice express-ing enhanced green fluorescent protein (GFP) on a universal promoter. After 4 weeks, the size of the thymus, the number and proliferation of thymocytes and ratios of different stage thymocytes were analyzed by immunohisto-chemistry and flow cytometry. The results showed that: 1) CD4 + CD8 + thymocytes were more sensitive to X-ray radiation -induced cell death than other thymocytes; 2) the proliferative capacity of CD4 + CD8 + thymocytes was higher than that of other thymocytes; 3) the size of the thymus, the number of thymocytes and ratios of thymo-cytes of different s the ratio of GFP-positive CD4 + or CD8 + thymocytes increased significantly indicate that the degree of sensitivity of thymocytes to X-ray radiation depends on their proliferative states and radiation impairs the maturation of donor-derived CD4 + CD8 + thymocytes in recipient thymus.