G0, G1, and mammalian cells and nuclei were shortly digested with either micrococcal nuclease or DNAse I, both before and after mild fixation, either before (G0) or after (G1) partial hepatectomy. Cells were Feulgen stained and examined by high resolution light microscopy. In metabolically active G1 nuclei, intranuclear DNA appears organized at least in two distinct domains, whereby the highly dispersed one is large enough to be detected at the resolution of the light microscope and appears preferentially attacked by limited DNAse I digestion. The action of the enzyme is readily apparent only in the nuclei that are first digested and then fixed. Spectroscopic characterization of the same nuclei reveals that the fixation causes a sizeable removal of proteins, mostly in the soluble chromatin subfraction. Results are discussed in terms of two control levels for gene expression and for higher order DNA structure.