The activity of carboxypeptidase A [EC 3.4.12.2] was inhibited by 3-phenylpropionate derivatives (p-aminocinnamate, 3-p-aminophenylpropionate and 3-p-acetylaminophenylpropionate), and to investigate its use as a ligand for affinity chromatography 3-p-aminophenylpropionate was directely and indirectly coupled to Sepharose 4B. carboxypeptidase A was adsorbed only on 3-p-aminophenylpropionate bound to the gel through p-phenylenediamine as a spacer. Carboxypeptidase A from pancreas was purified by a combination of this affinity adsorbent and ion exchange chromatography. The purified carboxypeptidase A had a homogeneity similar to that of a commercial product, as judged by disc gel electrophoresis. The carboxypeptidase activity of Pronase was slightly retarded on the gel column, but could not be separated from its caseinolytic activity. Angiotensin I-converting enzyme [peptidyl dipeptidy hydrolase, EC 3.4.15.1] obtained from hog kidney cortex was not bound to the gel.