An enzyme-immunoassay has been developed for the detection of myoglobin in human serum and urine which is specific, accurate, precise, and has a sensitivity of 3 ng/ml. When compared with radioimmunoassay, the enzyme-immunoassay gives markedly similar results. Sera from normal adults had a myoglobin concentration in the range 3–65 ng/ml, and 64% of the same group had detectable myoglobinuria (range 3–11.5 ng/ml). All of 8 patients with definite acute myocardial infarction had raised serum myoglobin levels (range 200-1125 ng/ml) either at admission or 4 h later. Myoglobin concentration returned to normal in 6 patients, and in the remaining 2 patients there was evidence of infarct extension. Urinary myoglobin excretion was variable. One patient with possible acute myocardial infarction had elevated serum myoglobin (413 ng/ml 4 h post admission) and 5 patients with no evidence of infarction had normal levels (15-53 ng/ml). The results suggest that detection of serum myoglobin by enzyme-immunoassay may be a valuable test in the early diagnosis of acute myocardial infarction.