An antiserum raised against chemically acetylated histone H4 was found to recognize the epitope epsilon-N-acetyl lysine. Affinity-purified antibodies were used to fractionate oligo- and mononucleosomal chromatin fragments from the nuclei of 15-day chicken embryo erythrocytes. Antibody-bound chromatin was found to contain elevated levels of acetylated core histones. On probing with sequences of alpha D globin, an actively transcribed gene, the antibody-bound chromatin was 15- to 30-fold enriched relative to the input chromatin. Using ovalbumin sequences as a probe, no enrichment was observed. The results demonstrate directly that transcriptionally active genes carry acetylated core histones.