A rapid procedure for the isolation of core particles from Chinese hamster ovary cells is described which permits measurements, usually at the day of their preparation. Particles of 145 f 5 base pairs, derived from interphase cells, will be compared with the analogue specimens from butyrate-treated cells, metaphase cells and a standard preparation from chicken erythrocytes. Butyrate cause an increase in the acetylation of histones H3 and H4, which induces alterations of the interhistone and histone-DNA interactions. Changes in the interhistone contacts, correlated to an extension of a-helical segments, lead to an altered accessibility of the H3 cysteine side-chains and to a different histone displacement by protamines. On the other hand, histone-DNA contacts are loosened in parts and this is particularly evident from the changes in the premelting region of a thermaldenaturation profile.In the structural element of chromatin two each of histones H2a, H2b, H3 and H4 form an octameric protein core which is surrounded by a 140-base-pair segment of DNA. These 'core particles' are linked by a variable segment of DNA to reveal a repeating subunit structure (nucleosome) [l]. Specific lysine side-chains within the highly basic aminoterminal regions of histone H3 (residues 9, 14, 18 and 23) and histone H4 (residues 5, 8, 12 and 16)