OBJECTIVE To optimize hematopoietic stem cell transplantation therapy and provide support for drug research by investigating the dynamic process of hematopoietic and immune system reconstitution after bone marrow transplantation(BMT)in mice.METHODS CD45.2+C57BL/6 mice were used as recipient mice and randomly divided into the normal control group and transplantation group,with 30 mice in each.The transplantation group was irradiated by a lethal dose of cobalt-60 rays.Bone marrow cells were prepared from CD45.1+C57BL/6 mice and transfused into recipient mice through the tail vein.Peripheral blood,spleens,lymph nodes,thymuses and bone marrow were collected at 1,2,4,8 and 16 weeks after transplantation.Blood routine examination was performed with peripheral blood and total cell numbers in suspensions of other organs were counted by an automated cell counter.Cell classification analysis of white blood cells in peripheral blood,cell suspensions of other organs was performed by flow cytometry.RESULTS Four weeks after BMT,the numbers of white blood cells and red blood cells in peripheral blood of recipient mice returned to the same level of or higher level than normal control(P<0.05).Although the number of platelets recovered significantly,it was still mark-edly lower than that of normal control until 16 weeks post BMT(P<0.05).In addition,the percentages of myeloid leukocytes and B cells in peripheral blood,spleens,lymph nodes,and bone marrow,as well as megakaryocytes and erythrocyte progenitor cells in bone marrow also returned to normal,and the majority of myeloid leukocytes and B cells were CD45.1+cells from the donors.Eight weeks after BMT,T cells in peripheral blood,spleens,lymph nodes,thymuses,and bone marrow of recipient mice returned to normal,and CD45.1+T cells were dominating.CONCLUSION The hematopoietic and immune reconstitution of recipient mice is nearly completed eight weeks after BMT.However,the reconstruction speed of different kinds of cells and the reconstruction status of same kind of cell in different organs vary widely.