In this experiment, the effect of bestatin on cyclophosphamide (CYC)-induced myelosuppression was studied to evaluate the hemopoietic activity. Bestatin administrated i.p. from -day 5 to -day 1, showed no influence on the peripheral blood pictures and bone marrow nucleated cell counts (NCC) in normal mice. On the other hand, in mice treated with CYC, 300 mg/kg, i.p. on day 0, bestatin accelerated the recovery from CYC-induced leukopenia and myelosuppression. The accelerated recovery of the total WBC count and bone marrow NCC were solely due to the enhanced granulocyte recovery. These findings indicated a promising effect of bestatin in the clinical management of myelosuppression induced by cancer chemotherapy.