Administration of a low dose of Epo induces a mild increase in peripheral erythropoiesis, whereas marrow erythropoiesis remains largely unaffected. Differential counts of (A) RBCs, (B) hemoglobin, and (C) hematocrit in PB 10 days after the first PBS or Epo injection (n = 17 per treatment). (D) Total BM cellularity. Fractionation of erythroid differentiation in BM using (E-F) CD71 and Ter119 or (H-I) CD44 and Ter119. The Ter119⁺/CD71^hi and Ter119⁺/CD44^hi fractions represent the proerythroblasts and basophilic erythroblasts, Ter119⁺/CD71^med and Ter119⁺/CD44^med compose the polychromatic and orthochromatic erythroblasts, and Ter119⁺/CD71^low and Ter119⁺/CD44^low contain enucleated reticulocytes and mature erythrocytes. (G) Numbers of granulocytes and macrophages, as determined by FACS using the markers Mac1, Gr1, and F4/80. Further fractionation of myeloid and erythroid progenitor subsets by FACS into (J) megakaryocyte progenitors (MkP), (K) myeloid progenitors (granulocyte macrophage progenitor), and (L) CFU-Es, Pre MegE, and Pre GM (left) and Pre CFU-E progenitors (right). (M) BM CFU-E colonies (n = 5). BM was collected 10 days after the first PBS or Epo injection (n = 12 per treatment if not stated otherwise). All data are presented as mean ± SEM.