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In this paper we wish to describe what we consider to be improved techniques that permit the isolation of mitochondrla and cell nuclei from the same liver homogenate. In the past it has been necessary to use different types of homogenates in isolating mltochondria and nuclei of acceptable quality. We also wish to demonstrate the importance of maintaining mitochondrial integrity during the isolation of nuclei (1, 2) and we offer evidence indicating that if the mitochondria are disrupted, an enzyme contained within them escapes and enters the nuclei, altering certain of their properties.The mitochondrla produced by the new technique not only are well preserved from the standpoint of morphology, but also fulfill two criteria that have been established for biochemically intact mitochondria (3), namely, that they have latent ATP-ase activity and that they require the presence of a phosphate acceptor for optimal oxidase 1 activity in the presence of those substrates, whose oxidation is coupled to phosphorylation. Previously, mitochondria which satisfied these two criteria have been altered morphologically (4,5). In this paper a detailed comparison is made of oxidase activities of mitochondrla isolated by the new procedure with the corresponding activities of those isolated in 0.25 M sucrose which are biochemically intact but morphologically altered. The ATP-ase activity of mitochondrla isolated by the new procedure is described in another publication (6).The new method has been applied mainly to rat liver, but it would probably work with other soft tissues, although in certain cases special steps to remove fiber would be necessary. The method fails when applied to Walker carcinoma

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