Staphylococcal nuclease digestion of purified chromatin from duck reticulocytes or calf thymus results in the production of a series of double-stranded DNA fragments of discrete molecular size, ranging from about 130 to 45 base pairs, which can be detected by polyacrylamide gel electrophoresis. Similar patterns of protected DNA fragments are obtained from limit digests of chromatin "reconstituted" from purified DNA and chromatin proteins. The results obtained with reconstituted material do not depend upon the origin of the DNA, which may be derived from a bacterial, viral, or homologous source. The specificity of the protective mechanism, therefore, resides in the structure of the bound histones, and probably not in any special nucleotide sequences present in the DNA. Removal of lysine-rich histones from chromatin before digestion results principally in disappearance from the digest of a DNA fragment about 130 base pairs long. Our preliminary results suggest that other elements of the digest pattern can be assigned uniquely to the remaining histone components. These results indicate that the binding of histones to DNA in chromatin involves a limited number of specific and very well defined contacts between protein and nucleic acid, which arise from structural properties of the histones.