A radioimmunoassay has been developed for the measurement of serum myoglobin in order to evaluate the time-course and frequency of myoglobinemia in patients with acute myocardial infarction. The method can detect as little as 0.5 ng of myoglobin and is not affected by hemolysis or storage of serum at -- 20 degrees C. Myoglobin was detected in all of 92 sera from normal adults and ranged between 6 and 85 ng/ml. Levels were markedly elevated in sera from 18 of 20 patients with acute myocardial infarction when samples were obtained within 12 h after hospital admission, the mean concentration being 380+/-53 ng/ml. Wehn the initial sample was drawn between 12 and 24 h after admission in another group of 20 patients with acute myocardial infarcts, the mean serum myoglobin concentration was 195+/-47 ng/ml, and 11 of these individuals had normal levels. Serial determinations performed on nine patients with acute infarction demonstrated that maximum myoglobin levels occurred within the first 8-12 h after admission and fell rapidly toward normal thereafter. The serum concentration of myoglobin in 21 additional patients admitted with chest pain but without acute myocardial infarction was 41+/-6 ng/ml. Radioimmunoassay of serum myoglobin appears to be useful and sensitive test for the early detection of myocardial infarction.