1. The reaction of dehydropiperidine carboxylic acid with p-dimethylaminobenzaldehyde was adopted as a basis for a quantitative, photometric determination of the compound.
2. A quantitative color development was attained in the presence of 0.056M of p-dimethylaminobenzaldehyde in 0.04M phosphate buffer of pH 7 by heating the reaction mixture for twenty minutes.
3. Under these conditions a number of common amino acids including α-aminoadipic acid, piperidine-2-carboxylic acid and piperidone carboxylic acid gave no significant color. The applicability of this method to the determination of Δ¹-piperidine-6-carboxylic acid, a cyclized form of α-aminoadipic acid-δ-semialdehyde, and of an intermediary compound in lysine biosynthesis from α-aminoadipic acid was demonstrated in both enzymatic system and intact cells of yeast.