Kinetics of the proliferative potential of purified human fetal BM CD34⁺ CD38⁻ cells in vitro. The growth factor cocktail included IL-3, IL-6, GM-CSF, SCF, and LIF. Data are presented as the total number of hematopoietic cells per well (mean of 15 wells) at each weekly time point. The standard deviation for the 15 wells at each weekly time point is less than 12% of the mean value. For comparison, the kinetic data of CD34⁺thy-1⁺ cells as shown in Fig 4 have been superimposed with the data obtained from CD34⁺ CD38⁻cells. (□), CD34⁺ CD38⁻; (◊), CD34⁺ thy-1⁺.