We have modified a limiting dilution liquid culture assay, used to quantify hematopoietic progenitor frequency, to simultaneously assess cellular proliferation and differentiation. The frequency of colonies from cord blood obtained with recombinant human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) in combination was comparable to cultures with IL-3 alone. However, IL-3 and GM-CSF in combination were synergistic for higher granulocyte proliferation than IL-3 alone, indicating that enhanced granulocyte production occurred via action of GM-CSF on progenitor cell populations already stimulated into proliferation by IL-3. Peak proliferation was evident at 4 weeks in culture, with metamyelocytes predominating; at 6 weeks, mostly neutrophils and eosinophils were present, and eosinophils were more numerous in cultures with IL-3. Increasing concentrations of erythropoietin (epo) in liquid culture with IL-3 or GM-CSF decreased absolute granulocyte yield while stimulating erythroid proliferation. The influence of epo on lineage morphology for cells plated in methylcellulose was markedly less evident by comparison, arguing against an inductive effect of epo to shift progenitor cell lineage. This liquid culture methodology may be a useful tool for preclinical screening of cytokines on human hematopoietic progenitor cells.