The role of acetylation on the conformation and association state of the inner core histone octamer isolated from HeLa cells was examined. Preparation of suitable quantities of pure acetylated and control inner core histones from HeLa cells required the development of a new preparative procedure. The results from size-exclusion high-performance liquid chromatography and sedimentation equilibrium studies indicated that acetylated inner core histones associate to species larger than the octamer and form a more stable complex. Circular dichroism studies demonstrated that the amount of a-helix increases with increasing association of the histones. Furthermore, acetylation results in an increase in the amount of a-helix, perhaps coupled through its effect on the association state. At high protein concentration and elevated temperature, the acetylated sample displays a greater increase in β-sheet content, relative to the control sample. This increase in 0-sheet content may be induced during the association of the acetylated sample to species larger than the octamer. There is a marked effect on the conformation of both acetylated and control inner core histones as a function of protein concentration, ionic strength, and temperature. The difference in conformational flexibility and association state of the acetylated vs. the control inner histone core may play a significant role in the control of transcription in the nucleus. © 1987, American Chemical Society. All rights reserved.