Distribution of xanthine oxidase activity in bovine tissues was investigated by biochemical assay and cellular localization of the enzyme was studied using a fluorescent antibody technique.XOD activity was detected in the homogenate of liver, spleen, duodenum, lung and kidney. Liver and spleen had at least five times higher activity. Fractionation of liver homogenate revealed the presence of enzyme activity only in the high speed supernatant fraction.Immunochemical properties of XODs in the soluble fractions from each organ homogenate were studied using antibody to milk XOD. The enzyme in these organs and that from milk revealed complete immunological identity.Immunohistochemically, in cryostat sections fixed with a strong fixative such as Carnoy's solution etc. the enzyme was localized as follows : Liver; in the cytoplasm of hepatic cells. Duodenum; in the apical portion of Brunner's gland epithelium. Kidney; in some of the epithelium of the proximal tubuli. Mammary gland; in the apex of alveolar epithelium and in the margin of alveolar spaces. Although the choice of the fixatives for immunohistochemical studies was empirical, possible reasons for lack of effect with fixatives employed in the present study are discussed.