The effect of a novel immunosuppressive agent, deoxyspergualin (DSG), on hematopoiesis in mice was studied with measurements of peripheral blood counts and assays of granulocyte-monocyte colony-forming cells (cfu-C) and spleen colony-forming cells (cfu-S) in bone marrow, during and after successive intraperitoneal administration of DSG. When DSG was administered at a strong immunosuppressive dose of 6.25 mg/kg daily for 15 days, mice developed significantly decreased peripheral blood counts and decreased bone marrow cells (BMC) during administration. After the completion of DSG administration a marked rebound in leukocytosis was observed and BMC returned to normal. In contrast, total cfu-C in the femur significantly increased during the DSG administration, and subsequently returned to normal. Moreover, total cfu-S in the femur were normally sustained in contrast with a decrease of BMC during the DSG administration. These findings suggest that DSG does not show generalized overt cytotoxicity against hematopoietic stem cells and freezes the ability of the stem cell in the proliferation or the differentiation.