The amine oxidases are enzymes which are of importance in the biological inactivation of amines naturally occurring in the animal body. They catalyse the oxidative deamination of a terminal amino group, free or substituted. The classification of these enzymes, which are widely distributed in animal tissues, is based on work carried out twenty years ago, when it was shown that there existed an enzyme called amine oxidase (or monoamine oxidase) which acts not only on adrenaline, tyramine and tryptamine, but also on many aliphatic monoamines, and a second enzyme called histaminase (or diamine oxidase) which is responsible for the oxidation of histamine and the aliphatic diamines, e.g. putrescine or cadaverine. A convenient source for the study of histaminase is the pig's kidney; the liver and many other mammalian tissues have served as a source of amine oxidase. It has since been shown that longchain aliphatic diamines are substrates of amine oxidase and not of the pigkidney histaminase. This work has been reviewed some time ago (Blaschko, 1952; see also Zeller, 1951).Little is known about the mechanisms of electron transfer in reactions catalysed by amine oxidases. The so-called 'mono' amine oxidase is not inhibited by cyanide, semicarbazide or carbonyl reagents, substances which are all potent inhibitors of histaminase.A few years ago an enzyme was found in the blood plasma of the sheep and the ox (Hirsch, 1953;Tabor, Tabor & Rosenthal, 1954) which is characterized by its high activity with spermine or spermidine as substrates. This enzyme is inhibited by inhibitors of histaminase, but it also acts on a number of monoamines which are typical substrates of amine oxidase. Thus this enzyme resembles amine oxidase in its substrate specificity but histaminase in its inhibitor specificity.The present studies of the amine oxidase of mammalian plasma arose from some observations on the oxidation of the anti-malarial compound, Primaquin * Present address: Institute of Physiology, University of Lund.