A carboxypeptidase capable of cleaving basic amino acids from synthetic peptide substrates is present in fresh human serum, and not in human heparinized plasma. Its activity is generated during the process of coagulation. Because of its instability at room temperature and at 37 degrees C, we named it "unstable carboxypeptidase" (carboxypeptidase U, CPU). The enzymatically active subunit of carboxypeptidase U was purified and exhibits a single band at 53 kDa on SDS-PAGE.