Serum Mn-superoxide dismutase (Mn-SOD) levels were determined in patients with acute myocardial infarction by an enzyme-linked immunosorbent assay (ELISA). The serial determinations performed on 29 patients with acute myocardial infarction indicated that Mn-SOD appears in serum in a biphasic manner; an early small elevation was observed at approximately 16.2 (+/- 7.3) h and a high elevation at approximately 108 (+/- 20.6) h after the onset of symptoms. A negative correlation was found between the maximum value of the late elevation of Mn-SOD and left ventricular function. Plasma levels of tumor necrosis factor-alpha (TNF-alpha) were also elevated in acute myocardial infarction. These facts suggest that TNF-alpha secretion from the infarcted area resulted in the production of Mn-SOD and that the induced Mn-SOD was released from myocytes into serum through damaged membranes, resulting in the late elevation.