The aim of this investigation was to isolate and characterize a neuropeptide-degrading carboxypeptidase from the muscular and mucosal layers of the human stomach. The carboxypeptidase was solubilized from membrane preparations of gastric muscle and mucosa using Triton X-100. The detergent-solubilized enzyme was purified to apparent electrophoretic homogeneity by affinity chromatography using lisinopril or potato carboxypeptidase inhibitor as an affinity ligand. The enzyme had an apparent molecular weight of 34,300 and was bound by concanavalin A and is thus a glycoprotein. The carboxypeptidase removed C-terminal leucine, phenylalanine, or tryosine residues from peptides including angiotensin I, [Leu5]enkephalin, kinetensin, neuromedin N, neurotensin, and xenopsin. It had an alkaline pH optimum and was inhibited by lisinopril, potato carboxypeptidase inhibitor, ethylenediaminetetraacetic acid, 1,10-phenanthroline, and 8-hydroxyquinoline. Immunoblotting indicated that the gastric carboxypeptidase cross-reacted with an antibody raised against a carboxypeptidase isolated from mast cells of human skin. The gastric carboxypeptidase released from gastric mast cells upon degranulation may act to degrade and inactivate neuropeptides in the stomach wall.