In 41 patients who underwent coronary bypass surgery, creatine kinase (CK)-MB mass concentration was repeatedly measured in serum during and after the intervention using a new two-site immunoenzymetric assay (IEMA). Serum CK-MB activity was determined with the use of four different techniques: immunoinhibition, immunoinhibition-immunoprecipitation, column chromatography and electrophoresis. Myoglobin (Mb) was also measured in each specimen by radioimmunoassay. In the 33 patients who followed a completely uneventful postoperative course, the cumulated CK-MB release was, on the average, 12.2-fold less than after acute myocardial infarction. The CK-MB peak concentrations using the IEMA were 33±3 μg/l (X±SEM) and occurred 6.4±0.5 h after the intervention was started; CK-MB levels had decreased to 2.9±0.4 μg/l at the end of the first postoperative day. The evolution of the CK-MB concentration was parallel to that of the enzyme activity. The serum Mb maximum concentrations (518±39 μg/l) were reached after 3.3±0.1 h. The other eight patients developed perioperative myocardial infarction (PMI); in this group, the cumulated CK-MB release was higher, and the serum CK-MB postoperative curves were of three different types. The patients with delayed CK-MB peaks (type I pattern) or sustained elevations (type III) of this isoenzyme also showed increased serum Mb levels at the end of the first postoperative day. The PMI patients with early (10 h) CK-MB elevations (type II) did not demonstrate abnormal serum Mb levels. The comparison of the results obtained 10 h and 24 h after the intervention for the different methods investigated indicated that: (a) serum CK-MB is superior to Mb for the monitoring of the patients undergoing coronary bypass surgery, and (b) among the various CK-MB measurements, the concentrations determined by IEMA are the best to differentiate the patients with PMI from the uncomplicated cases.