Using a methylcellulose cell culture technique, we studied the serial changes in erythropoietic precursors in the femur, spleen, and blood of mice under erythropoietic stimuli. Phenylhdrazine hydrochloride, in the dosage of 60 mg/kg, was injected into mice subcutaneously on days 0, 1, and 3, and mice were sacrified on days 0, 2, 4, 7, and 10 for assessment of erythropoietic precursors. Significant changes were observed for all hemopoietic organs in the number of erythrocytic burst-forming units (BFU-E) and erythrocytic colony-forming units (CFU-E). Only BFU-E were present in blood, and their maximal increase was noted on day 2. While marrow BFU-E continuously decreased, maximal increase of CFU-E noted on day 4. Splenic BFU-E and CFU-E increased until day 4 and declined subsequently. These observations suggest the presence of significant migration of BFU-E in mice under erythropoietic stimuli and stress the importance of studies on all hemopoietic organs in the assessment of murine hemopoiesis.