The specific activity of coumarin-7-hydroxylase was measured in liver microsomes from normal subjects and patients with liver disease. Liver specimens were obtained by needle biopsy and the microsomal fraction was separated by differential centrifugation. Its freedom from mitochondria was demonstrated by the absence of succinic dehydrogenase, a marker enzyme for mitochondria. Liver from healthy subjects showed variation in the specific activity of coumarin-7-hydroxylase from 0.16 to 0.65 nmol-mg-1-min-1, which is probably due to genetic factors. Patients with cirrhosis of the liver, chronic fatty hepatitis (chronic alcoholic hepatitis) and chronic active hepatitis showed a significantly lower mean hydroxylase activity. There was no significant difference in the mean level of hydroxylase between patients with subacute viral hepatitis or chronic persistent hepatitis and the normal controls.