The nature of protamine splitting proteinase, which was formed after treatment of human blood plasma by kaolin or silicone (aerosile), was studied. The activated enzyme did not exhibite the properties of thrombin and plasmin in reactions with specific substrates. Kyninogenic, TAME (N-tosyl-d-,l-arginine methyl ester)-esterase and protamine splitting activities were inherent in the proteinase; these properties enabled to group the enzyme with blood kallikreins. The purified preparations of human blood plasma kallikrein hydrolyzed protamine sulphate even at 10 mcg/ml concentration. alpha2-macroglobulin inhibited the protamine splitting activity of the plasma kallikrein.