We compared the analytical performance of three immunoassays used to rapidly determine creatine kinase (EC 2.7.3.2; CK) isoenzyme MB in serum: Dade's "Stratus," Corning's "Magic Lite," and Hybritech's "Icon QSR CK-MB." Performance criteria included precision, analytical sensitivity, sample stability, and analytical and clinical correlation of results for serum samples taken from healthy individuals, patients with suspected and confirmed acute myocardial infarction, and patients after coronary artery bypass surgery. We also examined 31 samples taken from patients in the emergency room suspected of myocardial infarction, to evaluate the potential of these assays for early diagnosis. Although these assays differ in the manner in which CK-MB is measured, and therefore have different procedural requirements, we conclude that they are equivalent in overall assay performance. None of these assays, however, is sufficiently sensitive for early diagnosis of myocardial infarction; therefore, results cannot be used by cardiologists in deciding whether acute thrombolytic therapy should be given. Other management decisions, such as the optimal utilization of intensive-care bed space, may justify using these assays on a "stat" basis.