The serum argino-succinate lyase (ASAL) activity was measured in preoperative and post-operative serum samples, from approximately 500 patients submitted to elective surgery. The results were compared with the determinations of 6 other enzyme activities on the same serum specimens. Serum ASAL elevations correlated highly with increases in serum alanine aminotransferase (ALT) activity, and were nearly twice as great. They also correlated well with serum aspartate aminotransferase (AST), but were approximately 2.7 times greater. It is confirmed that the serum ASAL activity is a sensitive and specific test for liver cell damage, suitable use in special conditions where high sensitivity is required.