Enriched populations of either normal human promyelocytes and myelocytes or blast cells were obtained by fluorescence-activated cell sorting with the monoclonal antibody WEM-G11. These populations were used to study the effect of pulse stimulation by purified recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) or cross-reacting purified murine granulocyte colony-stimulating factor (G-CSF). Maximal clone formation by promyelocytes and myelocytes was observed in 1-mL agar cultures stimulated continuously with 400 units of either CSF and in cultures of cells that were pulse stimulated by 3,200 units (or greater) of either CSF. Pulse stimulation by 800 units of GM-CSF or G-CSF generated 75% clone formation, and pulse stimulation by 200 units CSF gave 50% clone formation. The majority of clones formed by pulse-stimulated cells were only two cells in size; however, some clones were up to 15 cells in size after a single exposure to CSF. Clone formation was not observed in cultures of blast cell populations after a single pulse stimulation with GM-CSF or G-CSF.