Peripheral leucocyte counts and bone marrow examinations were made after intraperitoneal administrations of 10 mg Cyclophosphamide into guinea pigs for five consecutive days. The leucocyte counts were lowest on the 2nd day after the total dose of 50 mg had been injected, and returned to normal on the 8th day. Of the leucocytes component, the neutrophil counts most markedly decreased to 10% of their preinjection level, returned to the normal level on the 6th day, and revealed overshooting on the 8th day. Unlike the neutrophils, the lymphocytes decreased to 50% on the 2nd day and failed to recover fully even on the 8th day, showing a delay in recovery. Bone marrow examination in the diaphsis and epiphysis of the os longum after administration of Cyclophosphamide revealed similar findings as seen in peripheral leucocytes. The marrow megakaryocytes decreased remarkably on the 1st day after injection, and returned to normal on the 10th day. Myelocytes also decreased markedly on the 1st day, and the value became fairly normal by the 5th day. But there was a delay in recovery in the erythroblasts which showed a value of only 60% of the normal level even after 10 days. The fatty tissue of the bone marrow revealed changes which were contrary to that seen in other bone marrow cells.