This paper reports the effect of cyclophosphamide on the bone marrow hematopoiesis in the mouse. Cyclophosphamide 0.12 mg/g body weight was injected into the mice once and the observation lasted for 2 weeks. After the injection, peripheral leukocytes were reduced to the lowest level on day 4 and then increased higher than the control on day 7 to 14. The number of nucleated cell in the bone marrow was the lowest at the 48th hour and gradually became normal within two weeks. The pluripotent hemopoietic stem cells--CFU-S (colony forming unit-spleen) were depleted abruptly in 24 hours, then reproliferated exponentially to a peak on day 3, followed by a second decrease and came back to normal level on day 11 to 14. The changes of granulocytic progenitor cell CFU-D (colony forming unit-diffusion chamber) and CFU-C (colony forming unit-culture) were quite similar to that of CFU-S but their proliferation peak was on day 4. The peripheral leukocyte drop was slower and the return to normal was earlier than the hemopoietic cells. So the recovery of leukocyte count does not mean a real reconstruction of hematopoiesis. The bone marrow stroma observed by CFU-F (colony forming unit-fibroblastoid) assay and marrow microcirculation were also damaged and did not recover to normal during the observation. The bone marrow stroma and microcirculation showed a more serious damage.