Abstract 1. 1. Intravascular lysis of 24- and 48-hour-old thrombi in the femoral artery of rabbits and dogs by the use of human plasmin has been demonstrated. 2. 2. Human plasmin administered intravenously and intra-arterially is nontoxic in these animals. 3. 3. No significant difference is noted in the amount of plasmin needed for intravascular lysis whether it is administered intravenously or intra-arterially.