Employing the quantitative electrophoretic separation of cytosolic "LAP" activity (EC. 3.4.11.1), the authors investigated the clinical significances of cytosolic and total "LAP" activities in the sera of patients with several different hepatic disorders and elucidated the previously confusing results of aminopeptidase measurements in clinical laboratories. The dominant aminopeptidases measured in human sera by L-leucinamide as substrate are both cytosolic "LAP" and microsomal aminopeptidase (EC. 3.4.11.2). Total "LAP" activity means the sum of these two aminopeptidases. Different substrate specificities are observed in these two enzymes located in other subcellular fractions and different increasing rates of these enzymes are observed in various hepatic diseases. In cases of acute hepatic cell damage, cytosolic "LAP" accounts for a large percentage of total "LAP" of a patient's serum. On the other hand, in cases of elevation of hepatobiliary enzymes, microsomal aminopeptidase constitutes a large part of total "LAP" activity, and the ratio between cytosolic and total "LAP" activity is very low.