A sensitive radioimmunoassay for cardiac myosin light chain II (LCII) was developed, and changes of serum LCII levels were studied in experimental myocardial infarction in dogs. This radioimmunoassay employed an antiserum which was prepared in rabbits against canine LCII. In our assay, 0.2-5.0 ng of LCII were effectively measurable. In normal dogs, LCII concentration in serum was less than 20 ng/ml. The serum LCII level began to rise within 6 hr after ligation of the left anterior descending coronary artery, reaching maximum level at 3-5 days (40-320 ng/ml). In eight out of ten cases with coronary occlusion, LCII could be detected as long as 7 days after operation. In one sham-operated dog, LCII was detected at 2 and 3 days, but its concentrations were less than 30 ng/ml. When LCII was injected intravenously, it dissipated from the blood stream within 48 hr. The time course curves of serum LCII level had two characteristics that had not been observed in serum enzyme studies: 1) LCII level rose rapidly and stayed up during a long period after coronary occlusion, and 2) changes of serum LCII levels were biphasic in six out of ten dogs with coronary occlusion. These results, and our previous studies of synthesis rate of light chains, suggest that when a coronary artery is occluded LCII may first be released from a pool of uncombined LCII in myocardial cells, and then continuously liberated from cardiac myosin molecules. This radioimmunoassay can be expected to be useful when applied to clinical use.