It has been shown that indoleacetic acid (IAA) does not occur in developing grains of Hordeum vulgare L. (barley), but that an unidentified indolic compound does. This compound, designated 'A', was also found to be a product of the metabolism of exogenous IAA by barley. The expression of the gibberellic acid effect was delayed for at least 8 h if grains were imbibed in a solution of IAA, and during this time, the IAA was metabolised. The enzyme system involved could be peroxidase, which was active in the grains at all stages of their development and at maturity, and partially purified extracts of peroxidase were found to have considerable IAA oxidase activity.