The effects of differing dietary concentrations of sulfur (0.4, 1.8, and 3.9 g/kg diet) or Mo (0.3, 3, and 5 mg/kg diet) on Se utilization, were studied in Suffolk × Finn-Dorset ewes. Se concentrations and glutathione peroxidase (E.C.1.11.1.9) activity were measured in tissues of the ewes being repleted with Se or depleted of Se in the presence of the proposed antagonists. Evidence for an antagonism between dietary S and Se was found with an inverse relationship between dietary S concentrations and the Se concentrations of liver and wool, and glutathione peroxidase activity in the liver. Molybdenum (0.3, 3, or 5 mg) in diets of the sheep (0.01 or 0.03 mg Se/kg dry matter) had no significant effects on tissue Se concentrations or glutathione peroxidase activities. With an adequate supply of Se in the diet, there are probably no major effects of S on Se availability and metabolism in the sheep. Within dietary concentrations tested, dietary Mo does not influence utilization of Se by the sheep.