A newly developed, highly sensitive immunoassay for creatine kinase MB isoenzyme was evaluated in 68 patients with or without different types of ischaemic heart disease. Patients were classified on the basis of clinical criteria in four groups: no ischaemic heart disease, stable angina, unstable angina, and acute myocardial infarction. Enzyme concentration in patients with stable angina was the same (even during exercise) as seen in the patients without ischaemic heart disease. Patients with unstable angina, however, could be divided into two groups. One group showed clear evidence of severe myocardial ischaemia by serial changes and higher mean values of creatine kinase MB up to 40 hours after the onset of symptoms, whereas in the remainder values were stable and resembled those seen in the patients without ischaemic heart disease. The changes in concentration correlated with signs of repetitive ischaemic episodes deduced from continuous ST segment monitoring during the first 24 hours after admission. These findings indicate that patients with unstable angina are a heterogenous group. In some, severe and prolonged ischaemia can be detected by a serological assay with high sensitivity.