Frequent blood samples were drawn for determination of serum myoglobin, creatine phosphokinase (CPK), and the MB isoenzyme of CPK (CPK-MB) in patients with acute myocardial infarction (AMI). Significantly elevated levels of myoglobin were present 1.5 hours following onset of chest pain and predated elevations of CPK and CPK-MB by 3 hours. No evidence of the previously described "staccato" phenomenon was found. Due to very frequent blood sampling, a detailed picture of the evolution over time of the above indices was obtained. Significant differences were found in the biochemical profile of anterior wall infarction and diaphragmatic wall MI. A time-sensitivity curve (showing sensitivity of the assay at each time following onset of symptoms) was obtained for myoglobin, CPK, and CPK-MB. It appears that myoglobin is the most sensitive biochemical indicator of AMI in its early phase and since it decreases rapidly back to normal values, it can serve as an invaluable aid in the diagnosis of reinfarction and infarct extension. CPK-MB is a less sensitive indicator of MI but has the advantage of greater specificity.