Carboxypeptidase N has been purified 865-fold from pig serum. The enzyme has a molecular weight of approximately 315 000. In the presence of dodecylsulfate and mercaptoethanol, it dissociates into three subunits of Mr = 90 000, 50 000, 30 000, respectively. The native enzyme and the subunit of Mr = 90 000 contain carbohydrate; no carbohydrate is found in the subunits of Mr = 50 000 and 30 000. Trypsin transforms carboxypeptidase N into a form having a smaller molecular weight and enhanced activity.