Successive measurements of cardiac myosin light chain I (MLC), creatine kinase isoenzyme MB (CKMB), and the titer of antimyosin antibody (AMA) were performed prospectively in 19 patients following open heart surgery. Seven of these patients showed the postpericardiotomy syndrome (PPS). No differences in serum concentrations of MLC or CKMB were observed between the patients with and without PPS, and all patients in both groups had abnormal MLC values after surgery. However, only patients with PPS had significantly elevated AMA titers. The maximum AMA titer was significantly correlated with the severity of the effusion. These data suggest that PPS is unrelated to the severity of myocardial injury during operation. Furthermore, the AMA titer may be useful as one of the indicators for determining the patient's clinical condition.