C/EBPα-ER favors monocytic commitment of immature progenitors in 2 cytokine conditions. Marrow cells transduced with Puro, C/EBPα-ER, C/EBPαBR3-ER, or PU.1-ER(T) in IL-3/IL-6/SCF were lineage-depleted, cultured for 1 day at the indicated doses of E₂ (or in 4HT for PU.1-ER), and plated in methylcellulose in IL-3/IL-6/SCF or GM-CSF without E₂ or 4HT. The ratio of CFU-Ms to CFU-Ms + CFU-Gs was assessed at 7 to 8 days. The fold increase in percent CFU-Ms is shown for each condition as mean and SE from 3 determinations. P values, combining results in IL-3/IL-6/SCF and GM-CSF, are indicated.