1. 1. The IAA-destroying principle produced by Omphalia flavida is a soluble, oxidative enzyme. It can be exhaustively dialyzed, and precipitated with ammonium sulfate, without loss of activity. It shows optimal activity at pH 3.5, and at this pH its activity is not enhanced by 2, 4 dichlorophenol and is moderately stimulated by Mn ++. It is highly sensitive to HCN. Mn ++ prevents the enzyme activity from declining during the reaction. 2. 2. The IAA-destroying reaction is stoichiometric, consuming 1 mole O 2 and releasing 1 mole CO 2 per mole IAA destroyed. 3. 3. The enzyme does not act on indolepropionic or indolebutyric acid, nor on any of a wide variety of indole compounds except indoleisobutyric acid, which it oxidizes rapidly, and 5- and 7-hydroxyindoleacetic acids, which it attacks slowly. 4. 4. The ultimate reaction product is a mixture of at least four components, all of which contain the 2-carbon of IAA. Two give a positive Salkowski reaction, and several give yellow to red colors with the Ehrlich reagent. Neither indolealdehyde nor derivatives of o-aminoacetophenone nor quinoline can be present in appreciable amounts. The products are not phenolic and do not possess free aryl amino groups. At least one component is unstable and appears to undergo spontaneous slow oxidation and/or to give rise to polymeric material. The major constituents of the product appear to contain the oxindole nucleus. Some, but not all, of their properties agree with those of 3-methyldioxindole.