Growth factor stimulation results in phosphorylation of histone H3 at ser 10 and this correlated with expression of immediate early genes suggesting that this phosphorylation is associated with transcriptional activation. Although Western immunoblot analysis allows the detection of protein modifications in histones, in order to determine the localization of histones during different phases of cell cycle or during treatment of cells with different drugs we have to use immunohistochemistry. The protocol described here allows the detection of phosphorylated histones in tissue-cultured cells and tissue sections by fluorescent or bright-field immunostaining analysis. Here we used a serine 10 specific P-histone H3 antibody to determine the localization of this phosphoprotein in an asynchronously growing H4 glioma cell line and brain sections. It has been shown that long-term potentiation (LTP) is associated with gene transcription, and histone acetylation plays a major role in LTP formation (Wood et al., Learn Mem 13:241–244, 2006; Wood et al., Hippocampus 15:610–621, 2005; Alarcon et al., Neuron 42:947–959, 2004; Korzus et al., Neuron 42:961–972, 2004). Stimulus-induced phosphorylation of histone H3 at serine 10 has also been implicated in hippocampal neurons and striatal neurons (Li et al., J Neurochem 90:1117–1131, 2004; Crosio et al., J Cell Sci 116:4905–4914, 2003). Co-staining with a cell-specific antibody will allow us to determine the type of cells that show activation of histone phosphorylation in the brain.