Myoglobin and the enzymatic activity of creatine phosphokinase CK), MB-isoenzyme of CK (CK-MB), aspartate aminotransferase (GOT), alanine aminotransferase (GPT) and lactic acid dehydrogenase (LDH) were serially determined in 10 patients with acute myocardial infarction. Additionally the same parameters were assessed in 5 patients with angina pectoris for 24 hours after bicycle ergometry. 10 in-patients served as controls. Myoglobin was determined by radioimmunoassay and the other enzyme activities according to the current kinetic methods. Comparison of myoglobin with the enzymatic parameters showed that the myoglobin peak occurs 5.6 hours after the beginning of the sampling period, i.e. 7.3 hours earlier than CK and CK-MB and 11.6 hours earlier than GOT. In analogy to this finding the descending limb of the myoglobin curve was significantly earlier at a level of one third of the peak value, i.e. 8.2 hours earlier than CK-MB, 18.8 hours earlier than CK and 27.3 hours earlier than GOT. No signs of myocardial necrosis in terms of myoglobin or enzymatic activity could be detected after bicycle ergometry. It is concluded that myoglobin is a more sensitive parameter for assessment of the acute phase in patients with myocardial infarction than the usualy enzymatic parameters.