Standardization of aspartate aminotransferase (AST) determination is highly desirable for inter-laboratory comparison. Serum AST mean values for 20 patients suffering from viral hepatitis showed an inter-laboratory (n = 13) variation of 9.4%. Part of this variation was due to two laboratories using procedures without pyridoxal-5'-phosphate. A traceable AST value was assigned to an enzyme calibrator (EC) through the appropriate International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) primary reference procedure. The EC was commutable for seven routine methods. Common calibration using the EC reduced the inter-laboratory coefficient of variation (CV = 5.9% ) and allowed retention of a common reference interval for a set of routine procedures. Calibration made superfluous the expression of results in multiples of the upper reference limit, which increased inter-laboratory variation (CV = 18.5%). Furthermore, for 92% of patients, calibration with the EC allowed the correction of misclassifications when taking into account the reference interval of the reference procedure. Use of this EC could be proposed to complete the AST reference system.