It has been known that trypsin, an important proteolytic enzyme, can hydrolysis only lysyl and arginyl peptide, amide, and ester linkages (1, 2), Recently it has been shown that benzoyl-l-histidine ester (3) and n-fatty acid esters (4) are also susceptible to the action of the enzyme. Shapiro and Doherty (5) have briefly reported that ε-aminocaproic acid esters can also be hydrolysed by trypsin. The author has independently found in the course of a study on tryptic hydrolysis of poly-ε-aminocaproyl-α-alanines (6) that the ε-aminocaproyl peptides, amides and esters are hydrolysed by the action of trypsin. The present paper deals with detailed studies concerning the hydrolysis of the ε-aminocaproyl compounds by the action of trypsin.