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Earlier in this series of investigations, it was pointed out that succinic dehydrogenase and cytochrome oxidase are associated with the particulate components of cytoplasm (1), and attention was called to the similarity between these particles and the mitochondria which Bensley (2) separated from broken cell preparations by centrifugal means. Szent-Gy~rgyi (3) as well as Stern (4) had previously called attention to the fact that these enzymes are apparently attached to some macromolecular entity. The enzyme which we have called "coenzyme I-cytochrome c reductase" (5) is also in this category (5, 6), and Kabat (7) has shown that alkaline phosphatase is associated with the particulate matter.During this same period, Claude (8) has been perfecting the method of separation of the morphological components of cytoplasm by centrifugal techniques and has obtained discrete granules of material having "the general constitution of a phospholipid-r~onucleoprotein complex" with diameters ranging from 50 to 150 m#. The particles were considered to pre-exist in the original material and were shown to be similar in size and chemical composition to the particles of the Rous tumor virus.These independent observations provide no proof, however, that the enzyme systems include a ribonucleoprotein, since the particulate components might consist of a mixture of ribonucleoprotein particles and enzyme particles. Furthermore, there is thus far no evidence that any given particle contains more than one enzyme (9). Isolation of the enzymes in crystalline form does not provide an answer to the question since it seems likely that the highly active enzymes which have been isolated as soluble proteins of low molecular weight may not occur as such in the cell, but rather (4,7) that they are attached to the larger particles which Claude has shown to contain ribonucleic acid and phospholipid.Kunitz (10) has recently isolated ribonuclease in crystalline form and has demonstrated that it splits r~onucleie acid into smaller acidic groups without

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