Arginine carboxypeptidase (CPR) is a labile enzyme present in human serum which is unrelated to carboxypeptidase N. In this study we demonstrate that CPR exists in a precursor form in plasma and can be converted to the active form by trypsin and presumable trypsin-like enzymes. The trypsin-generated active form can not only cleave a small synthetic substrate, hippuryl-L-arginine, but can remove terminal arginine from bradykinin.