Endopeptidase 24.11 (enkephalinase), an enzyme known to be present in plasma and liver, is capable of metabolizing a substantial number of bioactive peptides. We measured plasma endopeptidase 24.11 activity in normal subjects and in patients with chronic hepatocellular disease or chronic cholestatic liver disease. The mean level of plasma endopeptidase 24.11 activity was 13 times higher in cholestatic patients than in controls or patients with hepatocellular disease (p < 0.01). Plasma endopeptidase 24.11 activity in patients correlated closely with traditional serum markers of cholestasis, including levels of alkaline phosphatase, gamma-glutamyltranspeptidase and aminopeptidase (p < 0.01 for all). However, plasma endopeptidase 24.11 activity correlated poorly with serum ALT aminotransferase level (p = NS), indicating that it is not a marker of hepatocellular disease. The lack of overlap between plasma endopeptidase 24.11 activity in cholestatic patients and noncholestatic liver disease controls suggests that this enzyme activity is a useful biochemical marker of cholestasis. In addition, because of the broad spectrum of peptides metabolized by endopeptidase 24.11, increased plasma endopeptidase 24.11 activity may contribute to the pathophysiology of the syndrome of cholestasis.