A carboxypeptidase which cleaves the C-terminal arginine or lysine from peptides was purified by a two-step procedure; gel filtration on Sephacryl S-300 and affinity chromatography on arginine-Sepharose. The activity increased 280% after the first step, indicating the removal of an inhibitor from the crude starting material. The activity in the crude seminal plasma eluted from the Sephacryl S-300 column with an apparent Mr 98,000 and after purification with an Mr 67,000, indicating that it binds to another protein in the crude seminal plasma. When analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, a single band at Mr 53,000 was seen which was converted to two smaller bands (Mr 32,000 and/or 26,000) after reduction. The seminal plasma carboxypeptidase has a neutral pH optimum, is inhibited by o-phenanthroline and by the inhibitor of carboxypeptidase B-type enzymes, 2-mercaptomethyl-3-guanidinoethylthiopropanoic acid, and can be activated by cobalt. The purified enzyme has a high specific activity (67.8 mumol/min/mg) with the ester substrate benzoyl (Bz)-Gly-argininic acid and readily cleaves Bz-Ala-Lys, Bz-Gly-Arg, and Bz-Gly-Lys. It also hydrolyzes biologically active peptides such as bradykinin (Km = 6 microM, kcat = 43 min-1), Arg6-Met5-enkephalin (Km = 103 microM, kcat = 438 min-1), and Lys6-Met5-enkephalin (Km = 848 microM, kcat = 449 min-1). The seminal plasma carboxypeptidase did not cross-react with antiserum to human plasma carboxypeptidase N; other properties distinguish it from the blood plasma enzyme as well as from pancreatic carboxypeptidase B and granular, acid carboxypeptidase H (enkephalin convertase). The carboxypeptidase could be involved in the control of fertility by activating or inactivating peptide hormones in the seminal plasma. In addition it could contribute to the degradation of basic proteins during semen liquefaction.