The activity and the Km (Michaelis-Menten constant) of serum aspartate amino transferase (AST) were investigated in normal subjects, in patients with recent myocardial infarction, and in patients with infectious hepatitis. AST activity was elevated in both groups of patients. While the Km value of serum AST was not affected in myocardial infarction, it was increased in infectious hepatitis when the total serum bilirubin was higher than 13 mg/dl. Thus, while an investigation of activity of AST in serum did not help in differential diagnosis of heart and liver diseases, determination of Km value was helpful in the diagnosis of infectious hepatitis.