Recent reports of neutropenia associated with the use of recombinant human erythropoietin (r-HuEpo) in preterm infants with the anaemia of prematurity have raised concern over the clinical use of this hormone. The present studies were undertaken to determine whether high-dose r-HuEpo has an effect on granulocyte production in vitro. The studies used a serum deprived, optimized semi-solid cell culture system to investigate the effect of lineage specific and non-specific granulocyte and erythroid colony stimulating factors on circulating peripheral blood granulocyte-macrophage colony forming units (CFU-GM), erythroid burst forming units (BFU-E) and multilineage colonies (CFU-Mix) from nine premature infants and seven healthy adults. CFU-GM were grown in the presence of interleukin 3 (IL3) 8 ng/ml, granulocyte-macrophage colony stimulating factor (GM-CSF) 20 ng/ml and granulocyte colony stimulating factor (G-CSF) 15 ng/ml alone and combinations of G-CSF with GM-CSF or IL3. The number, size and differentiation of CFU-GM colonies were then analysed in the presence and absence of high dose r-HuEpo (4 U/ml). High-dose r-HuEpo did not exert any significant modulatory effects on the number of CFU-GM colonies produced in the presence of IL3, GM-CSF and G-CSF alone or in combination. The number of cells within each CFU-GM colony did not change significantly, nor was there a significant change in the degree of differentiation. The combined number of BFU-E, CFU-GM and CFU-Mix colonies increased with r-HuEpo in both adults (1.8 x) and preterm infants (1.4 x), almost exclusively due to an increase in BFU-E derived colonies. Thus, no evidence was found for an r-HuEpo mediated redirection of multipotential haemopoietic stem cells into committed erythroid precursors at the expense of myeloid precursors.