Core histones isolated from normal and butyrate-treated HeLa cells have been reconstituted into nucleosome cores in order to analyze the role of histone acetylation in enhancing transcription factor binding to recognition sites in nucleosomal DNA. Moderate stimulation of nucleosome binding was observed for the basic helix-loop-helix factor USF and the Zn cluster DNA binding domain factor GAL4-AH using heterogeneously acetylated histones. However, by coupling novel immunoblotting techniques to a gel retardation assay, we observed that nucleosome cores containing the most highly acetylated forms of histone H4 have the highest affinity for these two transcription factors. Western analysis of gel-purified USF-nucleosome and GAL4-AH-nucleosome complexes demonstrated the predominant presence of acetylated histone H4 relative to acetylated histone H3. Immunoprecipitation of USF-nucleosome complexes with anti-USF antibodies also demonstrated that these complexes were enriched preferentially in acetylated histone H4. These data show that USF and GAL4-AH preferentially interact with nucleosome cores containing highly acetylated histone H4. Acetylation of histone H4 thus appears to play a primary role in the structural changes that mediate enhanced binding of transcription factors to their recognition sites within nucleosomes.