Serum myosin light chain levels were quantitated with radioimmunoassay in patients admitted to the coronary care unit. In this study there were 29 patients with acute myocardial infarction, 84 with chest pain but without myocardial infarction and 5 with chest pain but with preexisting electrocardiographic abnormalities that could not be interpreted using standard criteria. Values for human cardiac light chains in normal human sera averaged less than 2 ng/ml, and values in the 84 patients without myocardial infarction fell into this category. The 29 patients with myocardial infarction all had elevated light chain concentrations (average 10.9 +/- 1.3 ng/ml; range 4 to 35 ng/ml). In this study there were no false positive or false negative results. Use of this assay has permitted separation of patients with and without myocardial infarction. The assay for cardiac light chains provides a biochemical marker of high sensitivity and specificity that is useful in the diagnosis of infarction.