A method was developed for the isolation of macro- and micronuclei from Paramecium aurelia. This method utilized ionic and nonionic detergents to rupture the intact cells, calcium ions and spermidine were employed to protect the nuclei, and the nuclei were purified by centrifugation. Macronuclei consisted of 22% DNA, 10% RNA, and 68% protein. Micronuclei were composed of 9% DNA, 11% RNA, and 80% protein. DNA from both macro- and micronuclei had a density of 1.687 g/cc in CsCl and 1.417 g/cc in Cs2SO4. These values corresponded to G + C content of about 23%. The RNA of macronuclei was examined by gel electrophoresis, and two high molecular weight species were identified having molecular Weights of 1.3 x 106 and 2.8 x 106 daltons. Three syngens were studied, and in each case the conditions for isolation of the nuclei were the same and no differences were observed in the properties of the nuclei.

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