Fibrinolysin was used in 15 cases of myocardial infarction to evaluate its efficacy. Treatment began within 24 hours after the onset in each case, and results were compared with those obtained previously without fibrinolysin in 40 consecutive patients. Fibrinolysin was given intravenously in doses of 50,000 to 150,-000 units in 250 ml. to 500 ml. of 5% dextrose over a period of 4 to 6 hours. In none of the cases was the pain relieved by fibrinolysin therapy. The 2 groups did not differ significantly as to temperature, blood pressure, intensity and duration of pain, or mortality rate. Within a few hours after fibrinolysin therapy was begun, however, 7 of the 15 patients showed remarkable electrocardiographic changes. The changes were interpreted as a greatly accelerated regression toward the normal which, in the patients treated without fibrinolysin, had taken 2 to 4 weeks.