Xanthine oxidase from bovine thyroid glands was purified to apparent homogeneity when judged by analytical disc gel electrophoresis. The purification procedures include pancreatin digestion, butanol extraction, ammonium sulfate precipitation, calcium phosphate gel adsorption, ultrafiltration, calcium phosphate gel-cellulose column chromatography, gel filtration, preparative Sephadex G-25 column electrophoresis, and preparative polyacrylamide gel electrophoresis. The enzyme was enriched 1, 000-fold. However, its specific activity was markedly low as compared with highly purified milk enzyme. Thyroidal xanthine oxidase exhibited a low specificity for substrates and electron acceptors. The. kinetic properties of thyroid xanthine oxidase were found to be similar to those of the milk enzyme on the basis of Michaelis constants for common substrates.