It was shown that after treatment by Ca2+- and Mg2+-dependent DNAses and subsequent dosed ultrasonication the fractions of active and relatively inactive chromatins isolated from liver cell nuclei of rats differing in age contain all main types of histones, but differ considerably in the relative amounts of individual fractions of these proteins. In all age groups studied the proteins of relatively inactive chromatin are largely histones, while the amount of non-histone proteins is higher in active chromatin. In the course of postnatal development the amount of histones in both chromatin fractions is increased and that of non-histone proteins is decreased. This is probably due to heterochromatization of the chromatin complex in liver cells with ageing. In the course of postnatal ontogenesis the spectrum of non-histone proteins in both chromatin fractions is changed.