Activities of aspartate aminotransferase (AST) isoenzymes were determined in serial serum samples from 40 cases of acute myocardial infarction, and compared with activities of creatine kinase, CK-MB isoenzyme, lactate dehydrogenase, and alpha-hydroxybutyrate dehydrogenase for temporal changes. Cytosolic (soluble) AST (s-AST) and mitochondrial AST (m-AST) respectively increased 6.6 and 9.0 h after onset of chest pain. The median time at which serum m-AST activity peaked (15.8 U/L, range 6.4-53.5 U/L) was 47.8 h after the onset of infarction, 19.8 h later than the peak s-AST activity (171 U/L, range 53-517 U/L) and m-AST also disappeared from the serum more slowly than s-AST (p less than 0.001). Serum m-AST values were above normal for at least six days after the infarct. The ratio of m-AST to total AST in serum increased after myocardial infarction, being greatest (20%, range 11-32%) on the third day after onset. For individuals, peak activities of s-AST correlated well with total CK (r = 0.91) and CK-MB (r = 0.86) peak activities, indicating that s-AST also reflects the infarct size. However, m-AST correlated poorly with the enzymes commonly used in infarct diagnosis; it apparently provides different biological information.