B-cell potential is lost during primary culture on HS. CD34⁺CD38⁻ cord blood cells were cultured for 1 week in each of the four primary culture conditions shown and then switched to BSC on S17 stroma to measure maintenance of B-cell potential. At days 34 and 60 after switching, cells were obtained from BSC, counted, and analyzed by FACS for CD19 expression. Bars shown are means (±SEM) compiled from three independent experiments. LM, lymphoid medium; MM, myeloid medium-noHC/GF (see Materials and Methods). (▪), HS/LM; (□), HS/MM; (▧), S17/LM; (▧), S17/MM.