A considerable amount of work has been done on the identification and isolation of enzyme systems in plants responsible for biological oxidations. Various metal oxidases have been identified, such as polyphenol oxidase in leaves and other tissues (Nelson & Dawson, I 944), ascorbic acid oxidase in many tissues (Tauber, Kleiner & Mischkind, I935; Stone I937) and cytochrome oxidase (Hill & Bhagvat, I939; Brown & Goddard, I94I). In addition to these enzymes, which can bring about oxidation by molecular oxygen, other enzymes are known which catalyse oxido-reductions but are unable to react with oxygen. Lockhart (I939) identified a diaphorase in pea seedlings, and a number of dehydrogenases, capable of reducing methylene blue at the expense of their respective substrates, were shown to be present in seeds by Thunberg (I9I0, I9iI) and in other tissues by later workers. In addition to the occurrence of these various enzymes in both plant and animal tissues there is physiological evidence that many of the metabolic processes of plants follow the same general pathways as those known to occur in yeast and animal tissues (Neuberg & Kobel, I930; James & Bunting, I94I; Bonner & Wildman, I946). Following the demonstration (Whatley, I950) that coenzyme II (Co II) is present in the leaves of a number of plants it was thought to be desirable to look for enzyme systems in green leaves, perhaps similar to those in animal tissues, which would bring about the oxidation of substrates through the mediation of Co II. The experiments reported in this paper demonstrate the occurrence of two enzymic oxidation systems in extracts from the leaves of a number of plants. The two enzymes, an isocitric dehydrogenase and a diaphorase-like enzyme, were together able to bring about the reduction of methylene blue by isocitric acid with the aid of Co ii. A few of the properties of these enzymes have been investigated.