Proteases which inactivate bradykinin were partially purified from the fresh exsanguinated liver of rat, man, dog, guinea-pig, chicken, frog and snake. The enzymes which are present in the soluble fraction of the liver homogenates, were prepared by DEAE-cellulose chromatography, in 0.025 M tris-HCl, pH 7.4. The peptidase activity was eluted with 0.09 M KCl; and further purification was achieved by gel-filtration in Sephadex G-150. The kininases are present in the same range of activity in all studied preparations, and final specific activities are also comparable. The molecular weight of the enzymes, as determined by gel filtration, are in the range of 70,000-100,000. All preparations were completely inhibited by 10 mM PMSF and 1 mM Tos-PheCh2Cl; 10 microM 2-mercaptoethanol, and 1 mM Tos-LysCH2Cl do not affect the enzymatic activity. The major site for the cleavage of bradykinin is the Phe5-Ser6 peptide bond. The serine-peptidase is found in the liver of all vertebrates so far studied.