We have studied the reversible dissociation of core size DNA from chicken erythrocyte nucleosome core particles in solutions containing 0 X 1 M to 0 X 6 M-NaCl. Dissociation increases with increasing NaCl concentration, increasing temperature and decreasing particle concentration. At high particle concentrations, no free DNA is observed below 0 X 3 M-NaCl, whereas above 0 X 3 M-NaCl a lower limit of dissociation is reached. A theoretical analysis based on the migrating-octamer mechanism of Stein is in disagreement with his conclusions concerning dependence of core particle dissociation on particle concentration, but provides a good explanation for our observations, and those of others, using salt concentrations up to 1 M-NaCl. It appears that the core particle is not stabilized primarily by electrostatic interactions. DNA length is not critical for core particle stabilization. The conformation of remaining intact nucleosome core particles changes only moderately within the range of NaCl concentrations studied. Crosslinking by copper phenanthroline of the Cys110 histone H3 single sulfhydryl groups in the intact nucleosome core particle leads to a decrease in stability, yet essentially unchanged hydrodynamic properties are maintained at 0 X 6 M-NaCl, confirming conclusions derived from the behavior of the native core particles. Values for density increments of nucleosome core particles over a range of NaCl concentrations are also given. A method is described for studying binding of histones to nucleosome core particles in the ultracentrifuge by scanning at 230 and 260 nm.