The effect of the interaction between heparin and plasmin not only on fibrinolytic, caseinolytic and esterolytic activities but also on amidolytic activity, since plasmin has amidolytic or amidase activity, was investigated. Following were the results obtained from these investigations: 1. Heparin enhanced amidolytic activity of a fixed level of plasmin which was prepared by activating plasminogen with urokinase within the range from 2 to 64 units/ml of the final concentration of heparin. 2. Heparin also enhanced amidolytic activity of a fixed level of plasmin which was prepared by converting plasminogen with insolubilized urokinase. 3. Heparin did not enhance or inhibit fibrinolytic, caseinolytic and esterolytic activities of a fixed level of plasmin which was prepared by activating plasminogen with urokinase, in the fibrinolytic activity within the range from 0.032 to 125 units/ml, in the caseinolytic activity within the range from 0.0125 to 100 units/ml, and in the esterolytic activity within the range from 0.016 to 128 units/ml, of the final concentration of heparin respectively.