In order to indentify the effect of Astragali Radix(A.R) on cyclophosphamide(C.Y) induced leukopenia, A.R. extracts(EAR) were treated to mice orally, and blood sampling was done by periods. For the in vivo experiments, mice were divided into 4 groups, which treated EAR before, or after C.Y injection, or both, or none. Rapid normalization in the peripheral blood count of WBC, neutrophils, lymphocytes, RBC, and platelets observed in every EAR treated group regardless of the treatment periods of EAR. These studies suggest that, A.R. premedication can be effective in protection of bone marrow suppression during anticancer therapy.