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A double-antibody radioimmunoassay that specifically measures serum or urinary concentrations of human cardiac myosin light chains has been developed. The assay is both sensitive and specific for myosin light chain I. The antiserum used in the assay is capable of detecting 1--2 ng/ml of cardiac myosin light chain I and shows no cross-reaction to cardiac myosin, the heavy chains of myosin, actin, tropomyosin, or light chain II. Of 114 patients admitted to the Coronary Care Unit, 227 measurements obtained from 84 patients without myocardial infarction revealed that the serum light chain level of 2.0 +/- 0.18 ng/ml was not significantly different from that of a control group (2.1 +/- 0.17 ng/ml, n = 12). In contrast, 89 measurements from 30 patients with myocardial infarction evidence clinically, electrocardiographically, and by creatine phosphokinase determinations were found to have an average serum light chain I concentration of 10.9 +/- 1.39 ng/ml (P less than 0.001). Thus, the radioimmunoassay for human cardiac myosin light chain I provides a sensitive and specific marker for myocardial damage.

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