During the immediate-early response of mammalian cells to mitogens, histone H3 is rapidly and transiently phosphorylated by one or more unidentified kinases. Rsk-2, a member of the pp90rsk family of kinases implicated in growth control, was required for epidermal growth factor (EGF)-stimulated phosphorylation of H3. RSK-2 mutations in humans are linked to Coffin-Lowry syndrome (CLS). Fibroblasts derived from a CLS patient failed to exhibit EGF-stimulated phosphorylation of H3, although H3 was phosphorylated during mitosis. Introduction of the wild-type RSK-2 gene restored EGF-stimulated phosphorylation of H3 in CLS cells. In addition, disruption of the RSK-2 gene by homologous recombination in murine embryonic stem cells abolished EGF-stimulated phosphorylation of H3. H3 appears to be a direct or indirect target of Rsk-2, suggesting that chromatin remodeling might contribute to mitogen-activated protein kinase-regulated gene expression.