BLACKENING of the fruit of the banana may occur during cultivation, when the cause is usually of pathological origin, or during storage if ripening is not adequately controlled. The biochemical changes involved in the blackening of the fruit have received little study, although evidence for the existence of an enzyme system in the fruit of the banana capable of oxidizing substances of the catechol type was presented by Onslow1 in 1920. The existence of flavonoid compounds in the edible banana has been reported2,3. Robinson2 detected delphinidin in the hydrolysis products of the pulp of an edible banana while Simmonds3, who has described the anthocyanin pigments of the bracts of a number of banana species in detail, reported that the skin and pulp of fruit of the edible varieties contained large amounts of leucoanthocyanins. Indirect evidence was, however, obtained3 that the polyphenol oxidase system of the banana was unable to act on the leucoanthocyanins present to produce brown colorations. Recently, the presence of serotonin and related compounds in the fruit of the banana was reported by Waalkes et al. 5. The ability of these compounds to serve as substrates for polyphenoloxidase was not, however, explored.