Chick liver xanthine dehydrogenase was highly purified by preparative polyacrylamide gel electrophoresis at the final step of purification, which allowed removal of another contaminating, xanthine-oxidizing enzyme showing a molecular mass of about 380K daltons. Purified XDH showed a specific activity higher than 2, 500 units per mg of protein. On treatment with sodium dodecyl sulfate and 2-mercaptoethanol, XDH was split into two subunits (named as a and β) of different size in an equimolar ratio. The molecular weights of these subunits were estimated as 155K for a and 135K for β. In the form of sodium dodecyl sulfate-complex, subunit α tended to degrade into smaller peptides, whereas subunit β was relatively stable.