IL-1 signaling accelerates the differentiation of hematopoietic stem cells (HSCs) into myeloid cells in vitro by upregulating the transcription factor PU.1. IL-1 treatment of HSCs leads to increased expression of PU.1 and its target genes involved in myeloid differentiation. However, committed granulocyte-macrophage progenitors do not respond to IL-1 by increasing PU.1 expression, indicating IL-1 specifically affects the most immature HSC population to promote myelopoiesis.