Kunming mice were wholoe—body irradiated with 0.5~4.0 Gy X—rays. The effeet on different subset of thymocytes were analyzed by flow cytometry with fluorescent monoclonal antibodies 24 hour after irradiation. The results showed that the radiosensitivity varied in different stages of differentiation of the thymocytes. The order of radiosensitivity was: CD4,+, CD8,+,>,CD4,-,CD8,-,>,CD4,-,CD8,+,>,CD4,+,CD8,-,suggesting a relative rad ioresistance of the mature thymocytes. The D,37, values of CD4,+,CD8,+, CD4,-,CD8,-, CD4,-,CD8,+, and CD4,+, CD8,-, subsets were respectively 0.76, 0.92, 1.77 Gy and 3.67 Gy. The D,37, value of the thymocytes as a whole population was 0.84 Gy. The D,37, value of CD3,+, cell was 2.0 Gy.