Potential redundancy among members of the CCAAT/enhancer-binding protein (C/EBP) family in myeloid cells is indicated by the ability of C/EBPβ to replace C/EBPα in vivo, by the expression of granulocyte colony-stimulating factor receptor (G-CSFR) on C/EBPα^−/− cell lines, and by our finding that as with C/EBPα–estrogen receptor (C/EBPα-ER), either C/EBPβ-ER or C/EBPδ-ER can induce terminal granulopoiesis in 32D cl3 cells. To assess the consequences of globally inhibiting C/EBPs, we employed KαER, containing a Kruppel-associated box (KRAB) transrepression domain, the C/EBPα DNA-binding domain, and an ER ligand-binding domain. C/EBPs have a common DNA-binding consensus, and activation of KαER repressed transactivation by endogenous C/EBPs 50-fold and reduced endogenous G-CSFR expression. In 32D cl3 cells coexpressing exogenous G-CSFR, activation of KαER prevented and even reversed myeloperoxidase, lysozyme, lactoferrin, and C/EBPε RNA induction by G-CSF. In contrast, induction of PU.1 and CD11b, a gene regulated by PU.1 but not by C/EBPs, was unaffected. A KαER variant incapable of binding DNA owing to an altered leucine zipper did not affect 32D cl3 differentiation. Transduction of KαER into murine hematopoietic progenitor cells suppressed the formation of granulocyte colony-forming units, even in cytokines that enable C/EBPα^−/−progenitors to differentiate into neutrophils. The formation of macrophage and of granulocyte-macrophage colony-forming units were also inhibited, but erythroid burst-forming units grew normally. Thus, in 32D cl3 cells and perhaps normal progenitors, C/EBPs are required for granulopoiesis beyond their ability to induce receptors for G-CSF and other cytokines. One requisite activity may be activation of the C/EBPε gene by C/EBPα, as either C/EBPα-ER or C/EBPβ-ER rapidly elevated C/EBPε RNA in 32D cl3 cells in the presence of cycloheximide but not actinomycin D.