Clinical laboratories estimating enzyme activity in serum are using commercial lyophilized sera for four major purposes. These uses—as a standard and for intermethod, intramethod, and precision control—are segregated, and specifications for each deployment are examined in terms of requirements for the enzyme material: freedom from interfering or indicator enzymes and chromogens; high specific activity; inclusion of optimal cofactor concentrations; commutability, human properties and source; the presence of a single isoenzyme; and stability. The effects of serum matrix and variable assay conditions on the utility of enzyme materials are analyzed. Specifications differ for each enzyme material application. The compatibility of commercial lyophilized sera containing aspartate aminotransferase activity with several cited specifications is assessed