Kinetic study of the viability of CD34⁺ and CD34⁺CD38⁻ progenitor cells in serum-depleted medium. CD34⁺ (A) and CD34⁺CD38⁻ (B) cells were cultured at a density of 1 cell per well in 10 μL serum-depleted medium alone. After 44, 92, 116, or 140 hours of preincubation, 10 μL serum-depleted medium supplemented with a multifactor combination (IL-1, IL-3, IL-6, G-CSF, GM-CSF, CSF-1, FL, KL, Epo, and Tpo) was added to each well to yield predetermined optimal concentrations. Clones (≥5 cells) were scored after an additional 14 days of culture at 37°C and 5% CO₂ in air. The number of colonies observed at time 0 represents colony formation when the cytokine cocktail was added at the initiation of culture and thus is a control for optimal clonogenic growth in response to this cytokine combination. Results represent the means (±SEM) of the total number of clones formed per 180 CD34⁺ BM progenitor cells from two experiments (A) or 180 CD34⁺CD38⁻ progenitor cells from five individual experiments (B), with 180 wells scored per time point in each experiment.