Acetylation of histone residues regulates the expression of inflammatory genes and is controlled by the activities of histone acetyltransferases (HAT) and histone deacetylases (HDAC). Analysis of histone acetylation in human cells is limited by the large numbers needed to perform activity assays or Western blotting. We have used flow cytometry to investigate changes in HAT and HDAC activities at the single cell level and to investigate the effect of hydrogen peroxide (H(2)O(2)) on histone H4 acetylation and cell-cycle progression. Using an anti-acetylated histone H4 antibody we show that H(2)O(2) induced a time-dependent increase in histone acetylation that was maintained for 12h. This was associated with increased IL-8 production. H(2)O(2) also affected cell-cycle progression. HAT activity was found to be highest in G2/M and equivalent in G0/G1 and S phases of the cell cycle. These data show that detection of acetylated histone residues at the single cell level using FACs may be a powerful new tool for the analysis of modulation of cell proliferation and gene transcription.