Abstract: Objective To investigate the influence of bone marrow stem cell transplantation on immunosuppressed mice induced by radiotherapy. Me thods Female BALB/C mice were randomly divided into three groups. The mice in test group and in control group were irradiated with X-ray at dosage of 8Gy respectively, while those in blank group were not irradiated. After that, bone marrow stem cells 1x107 in 0.2ml normal saline were injected into mice by tail veins in test group, while 0.2ml normal saline (NS) was injected into mice by tail vein in control group. The peripheral blood was subjected to routine blood test on days 15, 25, 35 after transplantation, T lymphocytes (CD3~+, CD4~+, CD8~+) were determined by flow cytometry (FCM) on days15, 25, 35 after transplantation, and the concentration of mice peripheral blood immunoglobulins Ig M were measured by the ELISA method. All the mice were observed for survivals for 35 days. Re s ults The survival rates of mice 35 d after Hematopoietic stem cell transplantation were 80%, while 10% in control group. The leukocyte counts and Lymph% as well as CD3~+% in peripheral blood after transplantation increased as compared with those received NS (P <0.05) . However, CD4~+%, CD8~+% and serum immunoglobulin Ig M after transplantation decreased, which decreased remarkably on day 15 and begun to recover after that day. BMSC decreased Mono% and CD4~+/CD8~+in peripheral blood (P <0.05) .Conclus ion Bone marrow stem cell transplantation can repair the suppressed immunity of mice.