Glycogen phosphorylase isoenzyme BB mass release was studied in 20 patients undergoing coronary artery bypass grafting. In 16 patients with uneventful coronary artery bypass grafting, glycogen phosphorylase isoenzyme BB mass concentrations showed a significant, transient increase in the post cross-clamping period and decreased to baseline values within 20 hours (peak concentrations ranged from 12.7 micrograms/l to 47.5 micrograms/l, median 40 micrograms/l). One patient did not fulfil criteria for perioperative myocardial infarction, but clinical data indicated myocardial injury after aortic unclamping. In this patient only glycogen phosphorylase isoenzyme BB mass concentration and not creatine kinase isoenzyme MB catalytic concentration was increased, compared with uneventful patients. In 2 patients with emergency coronary artery bypass grafting for evolving myocardial infarction, glycogen phosphorylase isoenzyme BB mass concentrations, but not creatine kinase isoenzyme MB catalytic concentrations, correlated with clinical evidence of myocardial ischaemia. Our data indicate that glycogen phosphorylase isoenzyme BB mass concentration is a very sensitive laboratory marker of perioperative myocardial injury in patients undergoing coronary artery bypass grafting.