1. A thiol protease with properties similar to those of cathepsin B (EC 3.4.22.1) has been isolated and purified from human renal cortex. 2. This enzyme can generate renin activity from renin-suppressed and anephric human plasma at pH 5.5. 3. The renin activity thus generated is inhibited completely by antibody to purified human renal renin. This activation therefore represents the release of inactive renin.