Colony formation from cord blood CD34⁺ cells by varying concentrations of FL. 5 × 10² cord blood CD34⁺ cells were incubated in serum-containing culture, and the colonies were scored at day 14 of culture. FL alone induced only GM colony formation, and the number of the colonies increased in a dose-dependent manner, reaching a plateau at 100 ng/mL of FL. This concentration was assessed as an optimal dose.