Using a recently developed flow cytometric assay for the simultaneous measurement of cell surface antigens, DNA content and bromodeoxyuridine incorporation, we have for the first time determined the labelling index (LI), the duration of the S-phase (Ts) and the potential doubling time (Tpot) of purified CD34+ cells mobilized by G-CSF from 10 normal donors. Although CD34+ cells were not actively cycling immediately following purification, up to 5% could nevertheless traverse cell cycle without exogenous stimulation during the first 24 h of culture. In addition, it was possible to induce CD34+ cells to enter cycling by stimulation with haemopoietic growth factors (IL-3, IL-6 and SCF), resulting in median Tpot values of 18.2 d at 21 h, 7.7 d at 29 h, and 4.5 d at 37 h. Importantly, stimulation of CD34+ cells was seen almost exclusively within the CD38+ subset (mean Tpot value 2.8 d), whereas CD38- cells were not recruited into cycle (mean Tpot value 35.9 d). In conclusion, although cell cycle entry and progression can easily be induced in differentiated CD34+/CD38+ cells, immature CD34+/CD38- cells will remain dormant in most of the clinical and laboratory stimulation protocols hitherto employed. This assay can be used to obtain detailed cell cycle kinetics in leucocyte subsets in health and disease.