The amino acid sequence of the bovine xanthine oxidoreductase was determined by cloning and sequencing cDNA clones encoding the enzyme. Partial amino acid sequences corresponding to 54% of the total sequence were also determined from purified bovine milk xanthine oxidoreductase, showing identity with the translated cDNA sequence. The cDNA of 4719 nucleotides included a 5′ untranslated region of 96 nucleotides, an open reading frame encoding a xanthine oxidoreductase of 1332 amino acid residues, and a 3′ untranslated region of 624 nucleotides including two polyadenylation signals and a poly(A) tail of 74 nucleotides. The identity between the amino acid sequence of the bovine xanthine oxidoreductase and xanthine oxidoreductase from mammalian species was 86 to 90%.