A rapid method of determining creatine phosphokinase (CPK) isoenzymes using dithiothreitol, a reducing agent which activates the main fractions of the CPK molecule (MM and MB), was assessed in 68 subjects (34 with myocardial infarction, 18 with skeletal muscle lesions, 16 normal). In acute myocardial infarction, the mean peak of CPK-MB in serum increases to 13.3% of the total CPK (range 8.8% to 21%) from the 18th to the 24th hour after onset of symptoms, whereas in skeletal muscle lesions CPK-MB reaches only 1% of total CPK (range 0% to 2.9%), and in normal subjects CPK-MB is absent. This highly significant difference (p less than 0.001) makes it possible to distinguish accurately between myocardial infarction and skeletal muscle lesions (trauma, hematoma, myositis, hypothyroidism). By its sensitivity (100%) and specificity (97%) this rapid and inexpensive method finds its optimal sphere of application in emergency wards and coronary care units.