1. Known methods for isolating cell nuclei are divided into two classes, depending on whether or not the nuclei are capable of forming gels in dilute alkali or strong saline solutions. Methods which produce nuclei that can form gels apparently prevent the action of an intramitochondrial enzyme capable of destroying the gel-forming capacity of the nuclei. Methods in the other class are believed to permit this enzyme to act on the nuclei during the isolation procedure, causing detachment of DNA from some nuclear constituent (probably protein).

2. It is shown that heating in alkaline solution and x-irradiation can destroy nuclear gels. Heating in acid or neutral solutions can destroy the capacity of isolated nuclei to form gels.

3. Chemical and biological evidence is summarized in favor of the hypothesis that DNA is normally bound firmly to some nuclear component by non-ionic linkages.