This paper presents the first unified quantitative study of the effects of butyrate concentration upon (1) cell-cycle progression, (2) modification of all inner histones, (3) dephosphorylation of histone H1, and (4) enhancement of an H1-like protein (BEP) in CHO cells. Flow cytometric analysis shows that exposure to butyrate enriches CHO cultures in G1 cells and, at sufficient butyrate concentration, leads to G1 arrest. Additionally, butyrate alters the rate of cell-cycle progression through G2/M and through S. Two-dimensional polyacrylamide electrophoresis and radiolabeling in butyrate-treated cultures indicate the presence of at least one site of internal acetylation in histone H2A, four sites of internal acetylation in histone H2B, five sites of internal acetylation in histone H3, and four sites of internal acetylation in histone H4. Histone H2A is also appreciably phosphorylated, so that it is acetylated and phosphorylated at a total of up to three sites. The distribution of modified species for all the inner (core) histones has been quantified from two-dimensional gels by using the three different methods of analysis? (1) direct densitometry of excised portions of the gel, (2) scintillation spectrometry of 3H-labeled histones, and (3) microdensitometry of photographic negatives. At 15 mM butyrate, 26% of H2B is acetylated at three to four sites, 37% of H3 is acetylated at three to five sites, and 50% of H4 is acetylated at three to four sites. Histone H1 is dephosphorylated as a function of butyrate concentration, and the dephosphorylation can be correlated with an increased proportion of G1 cells in culture. There is also a significant increase in the cellular content of two other proteins when cells are exposed to butyrate. The increase in one of these, BEP, has been quantified as a function of butyrate concentration after 24 h of exposure to butyrate. BEP appears to be related to histone H1O [Panyim, S., & Chalkley, R. (1969) Biochem. Biophys. Res. Commun. 37, 1042] and to induced protein IP25 [Keppel, F., Allet, B., & Eisen, H. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 653]. The other protein (UP), which has a molecular weight of approximately 15 000, has not been identified. Butyrate induces a twofold increase in the cellular content of UP and a change in the distribution of UP molecular species.