To evaluate myocardial damage induced by coronary bypass surgery, to construct a reference data set in postoperative patients and to ascertain the impact of perioperative myocardial infarction on myosin release from sarcolemmal membrane we serially measured levels of serum fragments of myosin heavy chains by means of monoclonal antibody immunoradiometric assay (MYOSIN IRMA ERIA Pasteur). After surgery serum levels of myosin increased from third postoperative day and peaked on seventh day. Peak myosin levels did correlate with enzymatic activities of CPK (r = 0.69; p < 0.001) and of CPK-MB (r = 0.75; p < 0.001). Six patients suffered postoperative myocardial infarction: in these patients myosin levels were significantly different from those without infarctions (3651 +/- 1569 versus 699 +/- 206 ng/ml; p < 0.01). No correlation could be demonstrated between cardiopulmonary bypass time or aortic cross clamp duration. The rise of myosin levels observed in nearly all patients suggests a minor intraoperative damage to the contractile apparatus that sometimes is diagnosed as myocardial infarction and often does not meet clinical diagnostic criteria.