SUMMARY The phenomenon of plasma renin activation by add dialysis and prelncubation with trypsin was studied in normal human plasma. Activation of plasma renin by exposure to pH 3 3 was shown to require at least one dialysis step and could be inhibited by the presence of Trasylol, indicating the involvement of a protease in add actiration. Amniotic fluid exposed to pH 1.5 to destroy renin and renin substrate was also found to contain an enzyme capable of activating plasma renin. The Michaelis-Menten constant Km and the molecular weight of activated “renin” were found to be similar to those of normal plasma renin. Inactive renins or renin-like enzymes were partially purified from plasma by affinity chromatography on concanavalin A, precipitation with (NH4)tSO4 and isoelectric focusing. Trypsin and acid exposure gave similar results with regard to the activation of this zymogen, suggesting that trypsin and acid dialysis may increase plasma renin activity by the same mechanism.