Ligand-dependent transcriptional activation of retinoic acid receptors (RARs) is a multistep process culminating in the formation of a multimeric co-activator complex on regulated promoters. Several co-activator complexes harbor an acetyl transferase activity, which is required for retinoid-induced transcription of reporter genes. Using murine P19 embryonal carcinoma cells, we examined the relationship between histone post-translational modifications and activation of the endogenous RARβ2 promoter, which is under the control of a canonical retinoic acid response element and rapidly induced upon retinoid treatment. While histones H3 and H4 were constitutively acetylated at this promoter, retinoid agonists induced a rapid phosphorylation at Ser10 of histone H3. A retinoid antagonist, whose activity was independent of co-repressor binding to RAR, could oppose this agonist-induced H3 phosphorylation. Since such post-translational modifications were not observed at several other promoters, we conclude that histone H3 phosphorylation may be a molecular signature of the activated, retinoid-controlled mRARβ2 gene promoter.