Serum myoglobin concentration and creatine kinase activity were measured serially in 70 consecutive patients presenting within four hours of the onset of symptoms of suspected acute myocardial infarction. Of 36 patients with definite or possible myocardial infarction (WHO criteria), the serum myoglobin concentration was raised (greater than 85 micrograms/l) one hour after the onset of symptoms in 25% and at four hours in 89%. Creatine kinase activity was raised (greater than 140 U/l) one hour after the onset in 25% and at four hours in only 56%. Within 12 hours of the onset of symptoms the myoglobin concentration reached a peak in 83% and the creatine kinase a peak in only 14%. Within 36 hours the myoglobin concentration fell to normal values in 67% while creatine kinase activity fell to normal values in only 3%. Four hours after the onset of symptoms the serum myoglobin concentration distinguished easily those patients with myocardial infarction from those without, whereas when creatine kinase values were used the sensitivity was poor but the specificity high. From the combined results of the two studies and using a single measurement of serum myoglobin concentration at six hours from the onset of symptoms to predict the diagnosis in 114 patients with suspected infarction, the sensitivity was 93% and specificity 89%.