Administration of large doses of recombinant granulocyte colony-stimulating factor (rG-CSF) to mice results in diminished erythropoiesis. Hyporegenerative anemia does not occur in adult humans as a consequence of treatment with rG-CSF, but it is not clear whether this will be a problem in neonates. Because rG-CSF is currently being tested as a treatment for neutropenia in neonates, we assessed the possibility that such treatment will diminish their erythropoiesis. To do this, we added rG-CSF,in vitro,to clonogenic cultures of hematopoietic progenitors obtained from the bone marrow and liver of seven human fetuses and from the umbilical cord blood of five term and five preterm infants. The range of rG-CSF concentrations tested (0.1-10.0 ng/mL) included the peak concentrations measured in the blood of neonates receiving rG-CSF treatment on experimental protocols. Inclusion of rG-CSF in the cultures did not diminish clonal maturation of fetal erythroid (erythroid colony-forming and burst-forming unit) progenitors, nor did it reduce the number of normoblasts generated per erythroid progenitor cell colony. On the basis of these studies we predict that administration of rG-CSF to neonates will not result in down-modulation of erythropoiesis.Abbreviations: BFU-E,burst-forming unit-erythroid;CFU,colony-forming unit;CFU-E,colony-forming unit-erythroid;Epo,erythropoietin;rEpo,recombinant erythropoietin;G-CSF,granulocyte colony-stimulating factor;rG-CSF,recombinant granulocyte colony-stimulating factor;α-MEM,minimal essential medium, α modification