Using isopycnic centrifugation in metrizamide gradient, effect of ions and chromatin concentration on the buoyant density of chromatin was quantitatively examined. An elevation followed by gradual decline and secondary increase of the density occurred in accordance with increase in MgCl2 or NaCl concentration. Maximum density was observed at a concentration of these salts known to result in the condensation of chromatin. Release of protein occurred during the phase of density decline. The second increase in density is mainly due to the density increment of DNA in the chromatin. The density was dependent upon the concentration of chromatin in a band formed in the metrizamide gradient, while the density of free DNA and protein was not so greatly affected by their concentration. The density of chromatin in the presence of 0.14 M NaCl was less affected by the chromatin concentration than that in the absence of salt. Calculation of results indicates that grade of hydration of chromatin at concentrations lower than 400 microgram/ml in 1 mM Tris-HCl (pH 8.0) is higher than that expected from its DNA and protein components.