A review of methodology for determining aspartate aminotransferase (ASAT; EC 2.6.1.1), including recent national and international recommendations, indicates that standardization of methodology alone will not bring interlaboratory compatibility of ASAT results. We propose that an additional component to standardization is needed, namely, enzyme reference materials. Furthermore, we suggest that stable, well-defined ASAT materials from human sources are currently available. These primary reference materials and the state-of-the-art IFCC Reference Method for ASAT provide the basis for a unifying reference system for ASAT. Given such a reference system, we propose a practical way to promote compatibility of currently incompatible numerical results for ASAT through the use of one ASAT scale of units, the "International Clinical Enzyme Scale." This scale-unification concept would permit all current methods, instruments, and temperature choices to be used for ASAT determinations in the daily working laboratory. We present illustrative examples and demonstrate the unique ability of this concept to promote compatibility of the ASAT results from numerous laboratories using many different ASAT methods.