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.