Radioactive glycogen molecules have been used as passive probes to compare cavity systems within nuclei and isolated chromatin. Isolated chromatin was found to possess a narrow range of microspaces with mean effective diameters between 4.0 and 4.5 nm (40 and 45 A) depending on shape assumptions. Intact nuclei contained a far larger class of free spaces with average diameters in the order of 11.0-15.0 nm. This clearly shows that DNase-I (diam. 4.1 nm) can penetrate and occupy a large proportion of nuclear space even though this enzyme does not readily attack the undisturbed nuclear structure. A structure which simulated the pattern of penetrability and incorporated other known properties of chromatin was used to explain this DNase-I resistance of intact nuclei in terms of an ordered, compact, local structure interspersed by much larger spaces. A system for this local packing is suggested and the functional implications of this type of organization considered.