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Native, reassociated, and reconstituted core particles from chicken erythrocytes were compared by both biophysical and immunochemical methods. No significant difference between the three types of core particles could be demonstrated by electron microscopy, circular dichroism, or immunochemical analysis with antisera to histone H2B, H2A, and H3. Core particles were also reconstituted with calf thymus non‐acetylated H3, H2A, and H2B with either mono‐, di‐, or tri‐acetylated H4 isolated from cuttle ‐fish testes. The hyperacetylation of H4 did not significantly alter the biophysical characteristics of core particles but it induced several changes in their immunochemical reactivity. Binding to core particles of antibodies specific for H2A, H3, and for the IRGERA (synthetic C‐terminal) peptide of H3 was considerably decreased when di‐ or tri‐acetylated H4 was used for reconstitution, whereas binding of H2B antibodies remained the same. Our results suggest that the presence of hyperacetylated H4 within core particles leads to conformational changes that alter the antigenic determinants of several of the histones present at the surface of chromatin subunits. Since histone acetylation is correlated with the open structure of active chromatin, it may become possible to monitor the activity of chromatin by immunochemical methods.