Plasmin infusions were given to patients with advanced carcinoma on the assumption that the induced extravascular fibrinolysis would enhance body reactivity against the malignancy. Plasmin activity following intravenous infusion was demonstrated in the intravascular and extravascular spaces as well as in the circulating lymph. In the present study each of 11 patients received 4 or 5 infusions, after which followed a 4 to 5 wk. observation period, but significant signs of improvement failed to appear.