Two closely related but different aminopeptidases from bovine lung have been isolated and characterized. The first aminopeptidase, which removes the N-terminal arginine residue from L-arginyl-L-prolyl-L-proline, bradykinin, and des-[Arg^9]-bradykinin, has kininase activity; it has a pH optimum of 8.0, is stimulated by Mn^2+, and its molecular weight in dilute buffers is slightly greater than 240,000 daltons. The second aminopeptidase, which converts kallidin to bradykinin, has kinin-converting activity; it has a pH optimum of 6.8, is stimulated by Co^2+, and its molecular weight in dilute buffers is 240,000 daltons. The kininconverting enzyme is blocked from action when the N-terminal penultimate residue is proline.