Disrupted patterns of acetylation and deacetylation of core histones play an important role in silencing transcription of hematopoietic important genes in acute myeloid leukemia (AML). A thorough investigation of these mechanisms and the response to pharmacologic modifiers will provide a better understanding of the role of histone acetylation in leukemogenesis. We describe here an analytical approach that combines acid urea polyacrylamide gel electrophoresis (AU-PAGE), amino acid coded mass tagging (AACM), and mass spectrometry (MS) for the investigation of histone acetylation patterns. The combined approach was used to follow the dynamics of H4 acetylation in Kasumi-1 cells harboring the fusion gene AML1/ETO shown to aberrantly recruit histone deacetylases (HDACs). The histones in Kasumi-1 cells were labeled by growing the cells in media in which lysine was replaced with stable isotope-labeled lysine (Lys-D4). Labeled and unlabeled cells were treated with depsipeptide and analyzed at different time points (0, 4, 8, 12, 24, and 48 h). The cells were mixed, the histone was extracted, and acetylated H4 isoforms were separated using AU-PAGE before in-gel trypsin digestion. The digests were analyzed by MALDI-TOF MS. Peptides were identified by mass and isotope pattern. LC-MS/MS of Arg-C digests were also performed to verify the acetylation pattern for H4. The major pattern of acetylation was determined as follows: initial acetylation at K16, followed by acetylation at K12, and finally acetylation of either K8 and/or K5.