1. The effect of ribonuclease on various enzyme systems was studied as one approach to the problem of whether or not these enzymes are contained in macromolecules of ribonucleoprotein nature in protoplasm.
2. Ribonuclease inhibited CoI-cytochrome c reductase, succinic dehydrogenase, and cytochrome oxidase, all of which require cytochrome c in order to function. Ribonuclease did not act on cytochrome c.
3. Ribonuclease did not inhibit urease, xanthine oxidase, catalase, alkaline phosphatase, or adenosine triphosphatase under the conditions employed.
4. It was suggested that ribonuclease acted sterically by preventing contact between cytochrome c and its activating centers.
5. It was suggested that the enzymes inhibited may be contained in a ribonucleoprotein of macromolecular dimensions but that the enzymes not inhibited are not necessarily excluded from such a complex by the data presented.
6. Further evidence against the Szent-Györgyi theory of hydrogen transport was presented and discussed.