Dose response curves for rHuEpo (open circles) and Darbe (solid circles). Progenitor cells isolated from 12 to 24 week gestation fetal marrow were cultured for 10–14 days in increasing concentrations of Darbe (0–500 ng/ml) or protein equivalent concentrations of rHuEpo. The number of BFU-E increased significantly (p<0.01, 10 vs. 50, 100, and 500 ng/mL Darbe, and p <0.01, 0.05 vs. 0.5, 1.0, and 2 U/mL rHuEpo).