A sensitive radioimmunoassay of cardiac myosin light chain II (LC II) was developed, and changes of serum LC II levels were studied in experimental myocardial infarction in dogs. In our assay, 0.2-5 ng of LC II were effectively measurable. The time-course curves of serum LC II level after coronary occlusion had two characteristics which had not been observed in serum enzyme studies; 1. (1) LC II level rose rapidly and stayed up during a long period, 2. (2) changes of serum LC II levels were biphasic in some cases . © 1979.