Extracts from tissue cultures of crown-gall from Parthenocissus catalysed the destruction of indoleacetic acid in vitro with optimum activity at pH 4·5. The presence of two co-factors, Mn++ and 2,4-dichlorophenol, was necessary for this activity, which was found to be strictly aerobic. Chlorogenic acid, caffeic acid, scopoletin, ferulic acid, and gibberellic acid markedly inhibited IAA-destruction. Chlorogenic acid inhibition was reversed by the addition of H2O2. Chlorogenic acid was not oxidized by the IAA-destroying system and did not behave as a competitive inhibitor. The IAA-oxidase extract manifested peroxidase and phenolase activity with catechol and pyrogallol as substrates. However, this activity was greater at pH 7·0 than at the optimum for IAA-oxidase activity, pH 4·5. Further evidence of the existence of these two enzymes in the intact tissue was demonstrated by histochemical studies. In tissue slices, peroxidase activity was very high and widely distributed while phenolase activity was low and restricted to localized centres of the tissue.