Fas activation potently abrogates the growth of CD34⁺ progenitor cells in the presence of TNF. CD34⁺ cells were expanded for 6 days in SF medium supplemented with SFT3 (“Materials and methods”) in the absence or presence of CH11 (1 μg/mL) and TNF (20 ng/mL) as indicated. After 6 days, cells were counted and plated at a concentration of 500 cells per milliliter in cytokine-supplemented methylcellulose (“Materials and methods”). After an additional 10 to 12 days of incubation, CFU-GM (A) and BFU-E (B) colonies were scored as described in “Materials and methods.” Results (averages of 3 experiments; 2 with CB and 1 with BM) are presented as percentages of controls (cells expanded in the presence of SFT3; error bars represent SEMs).