Some properties of carboxypeptidase N purified from pig serum have been investigated. The amino acid composition resembles that of human carboxypeptidase N, but differs markedly from that of porcine carboxypeptidase B. The enzyme contains a mass fraction of 0.1 carbohydrates. Both ester and peptide substrates are hydrolyzed at the same site. Peptide substrates are hydrolyzed if lysine or arginine is the C-terminal amino acid, and provided that the penultimate amino acid is not proline. Argininic acid and epsilon-aminocaproic acid are good inhibitors of the enzyme.