Nuclei from butyrate-treated or control HeLa cells were separated into micrococcal nuclease sensitive and resistant chromatin. Those regions most sensitive to the nuclease, amounting to some 10% of the chromatin, consisted mainly of mononucleosomes with equimolar amounts of the inner histones H2A, H2B, H3, and H4, very little H1, and equimolar amounts of the two small high-mobility group (HMG) proteins, HMG-14 and -17. Both in butyrate-treated and in control cells, these nuclease sensitive monomers were some 5--7-fold enriched in DNA sequences which are transcribed into cytoplasmic polyadenylated RNA, while resistant monomers are depleted in the same sequences. Electrophoretic analyses of the transcriptionally active mononucleosomes revealed heterogeneity. Several subcomponents were resolved when monomers of butyrate-treated or control cells were electrophoresed at low ionic strength. Active monomer subcomponents differ in their molar content of HMG-14 and -17, in their content of H1 and A24, and in the length of their DNA. Some minor differences between nucleosomes of butyrate-treated and control cells were observed.