Arginine carboxypeptidase activity in human serum, measured with the hippuryl-L-arginine substrate, is about three times higher than in human plasma. This difference is much smaller when hippuryl-L-lysine is used as the substrate. When fresh serum is incubated at 30 degrees C, the arginine and lysine carboxypeptidase activity decreases until a stable activity, close to the plasma activity, is reached. This stable carboxypeptidase activity is attributed to carboxypeptidase N. The unstable carboxypeptidase differs from carboxypeptidase N in pH-optimum, esterase activity, substrate specificity, Co2+-activation and dithiotreitol activation. Blood cells are not responsible for the release of this enzyme during coagulation. No activator of carboxypeptidase N was detectable in human serum. Ion-exchange chromatography on DEAE-cellulose confirms the presence of two different molecular forms of arginine carboxypeptidase activity.