The enzyme micrococcal nuclease was used to examine the accessibility of chromatin extracted from brains of 13 patients with senile and presenile dementia of the Alzheimer type. Compared with chromatin extracted from brains of 8 patients without neurological signs or brain pathology and brains of 7 patients with nonAlzheimer dementia, Alzheimer chromatin was less accessible to this enzyme. Reduced accessibility was reflected by a reduced yield of mononucleosomes in comparison with dinucleosomes and larger oligomers. Both neuronal and glial chromatin were found to be similarly affected. The reduced yield of mononucleosomes from Alzheimer chromatin is not due to their increased breakdown, but is probably related to protein associated with the internucleosomal linker region that retards nuclease action. Dinucleosomes isolated from control and Alzheimer nuclease digests were examined for their protein complement. Three perchloric acid-soluble proteins situated in the histone H1 region of sodium dodecyl sulfate (SDS) gels were present in elevated levels in Alzheimer dinucleosomes. These results represent the first example of altered chromosomal proteins associated with a diseased state of the brain.