1. Homogenates of dog lungs freed from blood inactivate bradykinin. The bradykininase activity is present in soluble and particulate subcellular fractions.2. The fraction with the highest relative specific activity is that sedimenting between 8,700 g and 78,000 g. This fraction has been studied in more detail.3. There is evidence for two bradykinin inactivating enzymes in this fraction with apparent K(m) values of 7.5 muM and 120 muM.4. The bradykininase activity is not dependent on the concentration of chloride ion.5. The bradykininase activity is inhibited by EDTA, 2:3-dimercaptopropanol, nickel ions and some of the peptides from the venom of Bothrops jararaca and of Agkistrodon halys blomhoffii but not by 2-mercaptoethanol or N-ethylamaleimide.6. Angiotensin II in either 15 or 170 mM chloride ion is not an inhibitor of bradykininase activity but angiotensin I at either chloride concentration is an inhibitor. It is proposed that chloride is not necessary for binding of angiotensin I to converting enzyme but is necessary to ensure the correct orientation of substrate for hydrolysis to proceed.