SummaryThe activities of human plasma bradykininases (kininase I and II) have been investigated because of their importance as reactants in the kallikrein‐kinin system. The use of [14C] bradykinin as substrate allows radiometric analysis of the enzyme‐substrate reaction mixture. This is possible because separation of bradykinin from its hydrolysis products is achieved by chromatography over CM Sephadex C‐25. Use of this technique has allowed localization of bradykininase activities in plasma protein chromatograms and the separation and partial purification of kininase I and kininase II. Kininase I can be identified because it hydrofyses hippuryl‐L‐lysine, whereas kininase II converts angiotensin I to angiotensin II. The measurement of hydrolysis is simple and quantitative and has been used to demonstrate normal Michaelis‐Menten kinetics for both enzymes. The Michaelis‐Menten constants for the enzymes have been found to be 4·1 × 10‐7M (kininase I) and 0·94 × 10‐7M (kininase II).