A recently-described assay for xanthine oxidase was used for the estimation of serum xanthine oxidase (SXO) in patients with a variety of liver disorders. SXO was markedly increased in acute viral hepatitis, and in some patients with chronic aggressive hepatitis. In other forms of chronic liver disease or obstructive jaundice, there was little or no increase of this enzyme in serum. The results of a study of experimental liver damage in rats suggest that SXO is more sensitive than serum aminotransferases in detecting acute liver damage.