Enzyme preparations from Leucaena seedlings catalysed the formation of β-(5-methylisoxazolin-3-on-2-yl)alanine (MIA) by using 3-hydroxy-5-methylisoxazole (HMI) and O-acetyl-L-serine. Some properties of this enzyme are described. The β-substituted alanine synthases from Pisum and Citrullus seedlings could not catalyse the synthesis of MIA. The phytotoxic effect of HMI on rice seedlings is reduced by alanylation.