Chicken erythrocyte nucleus contains two classes of chromatin that differ in micrococcal nuclease susceptibility and solubility at physiological ionic strength.

Inactive chromatin of the chicken erythrocyte nucleus is shown to consist of two distinct classes (I and S). I chromatin (approximately 60% of the total genome) is insoluble at greater than 0.1 M ionic strength whereas S chromatin (approximately 40% of the total genome) is soluble at all ionic strengths studied (0.01--0.3 M). These chromatins are released from nuclei upon digestion with micrococcal nuclease by two separate parallel processes that do not have a precursor--product relationship to each other. Isolated I-chromatin fragments show a progressive reduction in size from 250 to approximately 50 nucleosome equivalents with increasing digestion times at 0-2 degrees C. Prolonged digestion of nuclei at 37 degrees C results in conversion of I chromatin to mononucleosomes that are insoluble at greater than 30 mM NaCl. Isolated S-chromatin fragments show a constant size distribution, independent of digestion time, that peaks at approximately 35 nucleosome equivalents. Prolonged digestion of nuclei at 37 degrees C results in the conversion of S chromatin to mononucleosomes that are soluble at physiological ionic strength. Both I and S chromatins contain a full complement of histones with no nonhistone proteins.