A simple and rapid colorimetric method, applicable to extracts of biological samples, is reported for measuring the toxic amino acid l-α-amino-β-methylenecyclopropylpropionic acid (hypoglycin). By fluorimetry, as little as 10 nmol (0.14 μg) can be assayed. The amino acid is made to yield formaldehyde by reaction with permanganate under defined conditions, and from the aldehyde, 3,5-diacetyl-4-dihydrolutidine is produced and measured.