The serum level of human ventricular myosin light chains 1 and 2 was determined by competitive ELISA assay in 56 patients, of whom 26 with acute myocardial infarction (18 with simple AMI and 8 with complex AMI), 10 with unstable angina pectoris and 20 healthy subjects. The serum level of human ventricular myosin light chains 1 and 2 (HVMLC1 and HVMLC2) in healthy subjects was of 6-10 ng/ml. Sensitivity of the assay was 10 ng/ml, and the working range was 10-100 ng/ml. In all AMI patients the HVMLC1 serum level exceeded 2-3 times the reference range within the first 24 hours after the onset of chest pain, the mean (+/- ISD) peak concentration was of 45.7 (+/- 13.6) ng/ml (simple AMI) and 64.22 (+/- 18.5 ng/ml) (severe AMI) 48 hours after infarction and it remained significantly higher than the normal value for another 7-10 days after the onset of infarction. The serum HVMLC2 concentration exceeded 10 ng/ml 48 hours after infarction, with a mean (+/- 1SD) peak concentration of 37.8 (+/- 10.4) ng/ml (simple AMI) and 53.8 (+/- 16.1) ng/ml (complex AMI) 90 hours after the onset of infarction and remained at high values for another 4-7 days. In two of ten patients with unstable angina pectoris LC2 increases above 10 ng/ml. The prolonged release of HVMLC1 and HVMLC2 in serum for 4-7 days after the onset of myocardial infarction may facilitate the retroactive detection of infarction, hence the extension of myocardium necrosis.