The relationship between myocardial infarct size and serum levels of cardiac myosin light chain II (LC II; 20000 daltons) was studied in 24 dogs with left anterior descending coronary artery occlusion. LC II in the serum was measured by the radioimmunoassay which we have recently developed. In our assay, 0.1--5.0 ng of LC II were effectively measurable. Serum LC II levels rose rapidly and stayed elevated long after coronary occlusion. Infarct size was determined by gross inspection. In 24 dogs, infarct size ranged from 0.3 to 41.7 per cent of left ventricular weight. LC II release was calculated by the formula of Shell and associates. Regression analysis showed good correlation between infarct size and LC II release (r = 0.78). Infarct size also correlated with maximal LC II level (r = 0.77), and LC II level 24 hours after coronary occlusion (r = 0.69). Detection of circulating LC II is a useful method since it can be applied to the diagnosis of acute myocardial infarction at the early as well as late stage and infarct size can be assessed by analysis of serum LC II levels.