The rapid, transient induction of 80-100 immediate-early (IE) genes upon mitogenic stimulation occurs irrespective of protein synthesis and is mediated by modification of existing proteins. Two mechanisms, not mutually exclusive, involving modification either of sequence-specific transcription factors or of structural chromatin proteins primed by pre-association with responsive effectors are conceivable. Here, we show that upon IE gene induction, the non-histone high-mobility-group protein HMG-14, but not the related protein HMG-17, becomes serine phosphorylated in its basic, amino-terminal region close to where it binds nucleosomal DNA. Phosphorylation, normally transient, occurs independent of transcription and is quantitative and prolonged during superinduction. Brief micrococcal nuclease digestion substantially releases HMG-14 from nuclei in the mononucleosome-bound state. Finally, mononucleosomes prepared from mitogen-stimulated, but not control, cells contain a mitogen-activated kinase that phosphorylates HMG-14 in vitro on the same site(s) as in intact cells. The association of HMG-14 and its mitogen-activated kinase with nuclease-sensitive mononucleosomes has implications for models of mitogen-stimulated IE gene induction.