The bromodeoxyuridine (BrdU)-Hoechst flow cytometric technique was applied to study the immediate cell kinetic response of highly purified human (h) bone marrow progenitor cells (CD(34+)-sorted fraction) to h granulocyte colony-stimulating factor (G-CSF) and/or h granulocyte- macrophage colony-stimulating factor (GM-CSF). The technique permits us to differentiate cycling from noncycling cells and to make a quantitative assessment of cell cycles after stimulation. Semisolid agar and single-cell liquid cultures were also performed to compare these initial events to the effects observed after 14 days of culture. The combination of G-CSF plus GM-CSF, acting synergistically in day 14 cultures, was found to have a subadditive effect in the first cell cycles, thereby indicating partial overlap of the different target cells. However, this combination accelerated transit through the cell cycle, as could be seen from the higher number of cells in the third cell cycle after 72 hours of stimulation. We conclude that, apart from the unresponsive cells, the CD34+ compartment consists of cells responsive to both G-CSF and GM-CSF, and cells responsive to either one of the CSFs alone, and that the combination of the two CSFs speeds up the cell cycle traverse rate for a significant fraction of the target cells that are initially responsive for both G-CSF and GM-CSF. The latter supports the hypothesis of an overlapping signalling pathway of G-CSF and GM-CSF.