Native chromatin has been characterized at different ionic strengths, according to different methods of preparation, by means of circular dichroism and ethidium bromide intercalation. For a more precise interpretation of these results, a series of analogous experiments on phagic DNA, as a function of its circularization, and on mononucleosomes have been performed. Results are discussed here in terms of high order chromatin structure, DNA supercoiling, and topological constraints.