During 'emergency' situations such as infections, host defense requires rapid mobilization of bone marrow granulocyte progenitors. 'Steady-state' granulopoiesis is absolutely dependent on the C/EBPα transcription factor, but the transcriptional mechanisms underlying emergency granulopoiesis remain unclear. Here we show that large numbers of granulocytes were generated from C/EBPα-deficient progenitors after cytokine stimulation in vivo. Cytokine treatment or fungal infection induced upregulation of C/EBPβ but not C/EBPα or C/EBPε transcripts in granulocyte progenitors, and C/EBPβ-deficient progenitors showed decreased emergency-induced granulopoiesis in vitro and in vivo. C/EBPβ inhibited proliferation less severely than did C/EBPα. These data suggest a critical function for C/EBPβ in emergency granulopoiesis, which demands both differentiation and proliferation of granulocyte precursors.