The increase of serum ASTm activity might reflect the severity of damage at the subcellular level of the myocardium. 50 patients with acute myocardial infarction (AMI) were observed. The mean peak ASTm activity was 34.34 +/- 34.60 IU/L and 48 patients (96%) greater than or equal to 9 IU/L (two times median value of normal subject). The peak time (36 h) came later and the duration (120 h) was longer than that of CK-MB. ASTm/ASTt ratio in groups of AMI, non-AMI heart failure and acute ictero-hepatitis was 0.25 +/- 0.10, 0.02 +/- 0.05 and 0.05 +/- 0.02 respectively. The former was significantly greater than other two groups (P less than 0.01). The activity of ASTm in AMI cases with heart function at I, II and III + IV (Killip classification) was 21.8, 40.2 and 76.2 IU/L respectively (F = 8.407 P less than 0.01) and it was 84.9 and 24.7 IU/L in the death and surviving groups (P less than 0.01). The result showed that the estimation of serum ASTm level was helpful to the establishment of diagnosis in the patients with AMI who were sent delayed to the hospital. It held special significance in evaluating the severity of myocardial damage, heart function and in predicting the prognosis of AMI.