Sera from patients with myocardial infarction and cardiac arrhythmias were analyzed for myoglobin concentration and the activities of total creatine kinase, creatine kinase isoenzyme-2, and lactate dehydrogenase isoenzyme-1 at the time of hospital admission and during the first few days of hospitalization. The nine patients with a final diagnosis of myocardial infarction had abnormally high values for total creatine kinase, creatine kinase-2, lactate dehydrogenase-1, and myoglobin. Myoglobin concentrations were highest on admission in six patients and on the day after admission in the other three patients. Creatine kinase-2 manifested maximum activity on the day after admission for all patients with myocardial infarction. Lactate dehydrogenase-1 did not reach maximal values until the second or third day after admission. The six patients with arrhythmias did not show any significant increases in creatine kinase-2 or lactate dehydrogenase-1. Myoglobin and total creatine kinase, however, were increased in the four patients who had received cardioversion. The specificity and diagnostic usefulness of these serum measurements are discussed.