Recovery capability of murine bone marrow progenitor cells after whole body irradiation with sublethal dose γ-ray was observed using the method of granulocyte-macrophage colony forming unit (CFU-GM) determination in vitro. The results indicated that normal numbers of CFU-GM and nucleated cells in bone marrow per femur were 4.89×10,4, and 2.4×10,7, respectively.The third day after irradiation both values described as above decreased to 0.8% and 4% of the normal respectively. The number of cells began to increase after the 5 th day.The numbers of nucleated cells in bone marrow and counts of WBC in peripheral blood changed in correspondence with the changes of numbers of CFU-GM.