The Dade Cardiozyme immunoinhibition procedure were evaluated for determination of creatine kinase isoenzyme MB (CK-MB) in 71 patients who were suspected of having had an acute myocardial infarction. Electrophoresis for CK-MB was also carried out. On the basis of diagnostic sensitivity and specificity for myocardial infarction, the Dade procedure for CK-MB was inferior to electrophoresis. In 11 patients for whom the time of infarction was known, normal CK-MB results were observed for 2 of them by immunoinhibition and electrophoresis during the the first 24 h, but subsequently abnormal CK-MB results could be detected by both methods. In some patients such data are not helpful for making a diagnosis in the first 24 h. The Dade procedure is easy to perform but lacks sensitivity in the region of low CK-MB activity, requires a very stable spectrophotometer, is imprecise and produces negative numerical results in patients without myocardial infarction.