An endogenous protease activating plasma inactive renin.

1. The protease inhibitors Trasylol and soyabean trypsin inhibitor prevented the activation of plasma inactive renin by acid. 2. N-Ethylmaleimide inhibited acid-activation to some extent but o-phenathroline had no effect. 3. Acid-activation of the inactive renin in human plasma is mediated by a serine protease.