Xanthine oxidoreductase (XOR) was purified from goats' milk. The u.v.–visible absorption spectrum was essentially identical to those of the corresponding bovine and human milk enzymes and showed an A280/A450 ratio of 5·20±0·12, indicating a high degree of purity. Like bovine and human milk XORs, enzyme purified from goats' milk showed a single band on SDS-PAGE corresponding to a subunit with approximate Mr 150000. On Western blotting, mouse monoclonal anti-human XOR antibody cross-reacted with purified caprine and bovine XORs. The specific xanthine oxidase activity of goats' milk XOR, however, was very much lower than that of bovine XOR, although NADH oxidase activities of XOR from the two sources were similar. In these respects, the caprine milk XOR mirrors the human milk enzyme, in which case the kinetic effects have previously been attributed to relatively low molybdenum content. The molybdenum content of goats' milk XOR also was shown to be relatively low, with 0·09 atoms Mo per subunit, compared with 0·55 atoms Mo per subunit for the bovine enzyme. A parallel purification of human milk XOR showed 0·03 atoms Mo per subunit. The possible physiological significance of the low molybdenum content of the caprine milk enzyme and of its correspondingly low enzymic activity is discussed.