Several naturally occurring amino- and imino-acids having structures closely related to those of some protein constituents have been tested for growth inhibitory activity upon germinating seeds, particularly on mung bean (Phaseolus aureus). Azetidine-2-carboxylic acid, a structural analogue of proline, and a-(methylenecyclopropyl)glycine, related to leucine, produced a marked inhibition of radicle growth, and quite low concentrations of the former were lethal to the seedlings. The potent action of azetidine-2-carboxylic acid was shown to depend upon its incorporation into plant proteins, where it replaced an equivalent amount of proline. Growth inhibition could be reduced by providing proline to the seeds together with the analogue. The effect upon protein structure and enzyme activity resulting from the incorporation of azetidine-2-carboxylic acid is discussed.