The author, discussing theories regarding the nature of the action of oxidizing enzymes, distinguishes between aerobic oxidases, or those which act only in the presence of air and not in the presence of the usual H-acceptors, and the dehydro-genases, or those which act in the absence of O provided a suitable H-acceptor is present. Whereas, according to the author, it is generally accepted that the latter act as catalysts by activating H in the substrate, it still remains to be demonstrated whether the former catalyze oxidations by activating 0 or H. The following types of aerobic oxidases are then discussed separately and in detail so far as tests for their presence and theories regarding the nature of their catalytic action are concerned[long dash]peroxidases, tyrosinase, laccase, polyphenolase, dopa-oxidase and indophenol oxidase. From the oxidation of phenols by aerobic oxidases, Raper concludes that the failure of the enzyme-phenol combination to reduce methylene blue suggests that increased H activation does not take place, whereas the fact that the phenol is rapidly oxidized in the presence of the enzyme and O indicates that the O has been activated by the enzyme. He therefore concludes that both methods of action of oxidizing enzymes may be of use in living tissues. The bibliography includes 83 references.