1. A modified Behrens procedure is described for the isolation of nuclei from avian erythrocytes and from the liver, kidney, thymus, pancreas, heart, and intestinal mucosa of the calf or horse.
2. The purity of these nuclei has been established by staining reactions, enzyme studies, and immunological tests for serum proteins.
3. Evidence is presented to show that a transport of cytoplasmic proteins into the nucleus does not occur during the isolation.
4. Nuclei prepared in non-aqueous media contain considerably more protein and a very different enzyme composition from that observed in nuclei prepared by "homogenization" techniques in dilute citric acid.
5. The suitability of nuclei prepared in organic media for the study of intracellular enzyme distribution is discussed.