Rapid radioimmunoassay of myoglobin is developed. The complete procedure requires about 60 min. The procedure might be used in routine laboratory practice. Content of myoglobin constituted from 14 to 56 ng/ml in blood serum preparations of 50 donors. In blood sera of 120 patients with acute myocardial infarction concentration of myoglobin was increased up to 90--6,000 ng/ml within 2-6 hrs, returning to the normal values within 38-48 hrs after the heart attack.