Efficient expansion of ELTC-CFC requires very high concentrations of early acting cytokines. Fifty (LTC-IC assay) and 1,000 (ELTC-IC assay) human CD34⁺CD38⁻cells were cultured in SF-medium in the presence of KFM at low (50 ng/mL) and high (250 ng/mL) concentrations. After 12 days of stroma-free culture, expanded cells were transferred to irradiated stroma cocultures. After either 5 or 12 weeks, the number of CFC produced was evaluated in methylcellulose. Results are the means of triplicate wells per group from 4 separate experiments.