Effect of dexamethasone on the growth rate of normal human erythroid progenitor cells in serum-free medium. CD34⁺ cells were incubated in IDMEM supplemented with either 15% fetal calf serum (FCS, ▪) or 20% BIT-9500 (BIT, serum substitute, ▵). All cells received SCF (50 ng/mL). The medium concentration of Epo was maintained at 5 U/mL in FCS and BIT while it was reduced to 1 U/mL in BIT/Dexamethasone (Dexa)-medium (○). In addition, the latter medium contained 1 μmol/L of dexamethasone. For detailed composition of media see Materials and Methods. Starting at day 5, the cells were split 1:2 on each successive day, always adding ½ volume of fresh medium. Cells grown in the absence of dexamethasone entered terminal differentiation after day 8.