By differential sucrose gradient centrifugation of pig kidney chromatin in the presence or absence of Na-EDTA and under varying ionic strength conditions, three nucleosome-like subpopulations with different buoyant densities can be obtained. These particles, on the basis of their histones and HMG protein pattern, of the 5-methylcytosine level of their DNA and of the RNA polymerase activity associated with them, can be considered as originating from chromatin fractions differently involved in gene expression. Two-dimensional electrophoresis of the tightly-bound non-histone proteins shows a distinct pattern for each subpopulation, such protein components being notably present in restricted numbers but in high amounts in the subpopulation which was apparently derived from condensed heterochromatin.