The nature of the structural changes induced by histone acetylation at the different levels of chromatin organization has been very elusive. At the histone level, it has been proposed on several occasions that acetylation may induce an alpha-helical conformation of their acetylated N-terminal domains (tails). In an attempt to provide experimental support for this hypothesis, we have purified and characterized the tail of histone H4 in its native and mono-, di-, tri-, and tetra- acetylated form. The circular dichroism analysis of these peptides shows conclusively that acetylation does increase their alpha-helical content. Furthermore, the same spectroscopic analysis shows that this is also true for both the acetylated nucleosome core particle and the whole histone octamer in solution. In contrast to the native tails in which the alpha-helical organization appears to be dependent upon interaction of these histone regions with DNA, the acetylated tails show an increase in alpha-helical content that does not depend on such an interaction.