The dipeptidylcarboxypeptidases that degrade leucine enkephalin in human and guinea pig plasma were studied by kinetic and chromatographic techniques. 2. The extremely rapid degradation of enkephalins in Cavia plasma seems to be caused by both increased activity of enzymes and reduced role of inhibitors. 3. The increased role of dipeptidylcarboxypeptidases in Cavia as compared to Homo appears prevalently caused by the presence in the former species of a considerable number of very active enzymes. 4. The sum of these data indicates the existence of noticeable intraspecific differences either in peptide-degrading enzymes present in plasma, or in plasma peptides, or in both.
