Objective:To investigate the effect of bone marrow mesenchymal stem cells (mMSC) infusion on the reconstruction of hematopoiesis function in mice with critical irradiation injury.Methods: Female C57BL/6 mice irradiated with 8 Gy 60Co γ-rays were randomly divided into control group and mMSCs-treated group.Mice in the mMSCs-treated group received mMSC of male C57BL/6 mice.The peripheral leukocyte,platelet,and erythrocyte counts,the pathological lesions,granulocyte-macrophage progenitors (CFU-GM),and sex chromosome ratio were all examined.Results: All the animals in the control group (n=6) died of hematopoiesis exhaustion 20 days after irradiation.The peripheral leukocyte of mMSC-treated group decreased greatly after bone marrow transplantation and quickly recovered 2 weeks later; till 28 days after transplantation,it recovered to 60% of the pre-irradiation level.mMSCs infusion promoted the quick recovery of the nucleated bone marrow cells and CFU-GM,improving the marrow morphology.The transplanted mMSC could be detected in the recipients 42 days after transplantation.Conclusion: Solo-injection of mMSCs can enhance hematopoietic reconstitution in mice with critical irradiation injury. The period of imbedding is related to the dose of infusion.