Expression of tissue- and development-specific genes is coordinately regulated during maturation of hematopoietic precursor cells toward functional, end-stage peripheral blood (PB) cells. To study the expression and methylation of several myeloid-specific genes during in vitro differentiation of normal hematopoietic progenitor cells, we used a model of CD34+ selected PB progenitor cells (PBPCs). PBPCs from six patients with solid tumors were recruited by standard-dose chemotherapy and subsequent administration of recombinant granulocyte colony-stimulating factor (G-CSF). PBPCs were collected and CD34+ cells selected by immunoadsorption columns using a biotinylated anti-CD34 monoclonal antibody. Enriched cells contained between 78% and 90% (median, 84%) CD34+ cells as determined by fluorescence-activated cell sorting analysis. Cell preparations were cultured in the presence of interleukin-1 beta (IL-1 beta), IL-3, IL-6 and stem cell factor and with or without G-CSF for various time intervals up to 20 days. Genes for CD34 surface antigen, lysozyme (LZM) and myeloperoxidase (MPO) were examined by RNA and DNA analyses. A rapid and early downregulation of CD34 transcripts was observed, with concomitant, time-dependent upregulation of expression of both the LZM and MPO genes. These effects were enhanced in the presence of G-CSF. Analysis of the DNA methylation status at key sites within these genes showed a pattern of differentiation- and expression-associated demethylation of the LZM gene, which was also enhanced by G-CSF, and constitutive and unaltered demethylation at key regions of the CD34 and MPO genes. In conclusion, the genes for CD34, LZM, and MPO are regulated during in vitro culture of very immature PBPCs in the presence of stem cell factor, IL-1, IL-3, IL-6; their effects are enhanced by G-CSF.