Summary Treatment of Avena coleoptile sections with IAA results in a rapid decrease in the level of lipid peroxidation (LP), as measured by the thiobarbituric acid reaction for malondialdehyde (MDA). The response is specific for active auxins, is nearly saturated by 10 −6 M IAA and occurs even when turgor is reduced by 0.1 M mannitol. About half the reduction in MDA occurs within 1 min after addition of IAA to intact tissues; addition of IAA directly to homogenates, however, has no effect. Homogenates prepared from auxin-pretreated tissues, but not control tissues, continue to produce MDA over at least a six-hour period. This effect of auxin on LP is one of the most rapid biochemical responses to auxin known, and suggests that LP might alter the properties of membranes and thus influence cell enlargement.