Sulfhydryl oxidase (SOX) is present in human milk and in milk from all species that have been studied. The pH optimum of human milk SOX is in the neutral range between 7.0 and 7.5. Human milk SOX is stable in an acid environment: 50% of its activity remains after 1 h at pH 2.5. Acid stability is also characteristic of y-glutamyltranspeptidase, another membrane-bound enzyme in skim milk. SOX is resistant to pepsin (4000 U/ml), trypsin (50 μg/ml), chymotrypsin (200 μg/ml), and to trypsin plus chymotrypsin (25 μg each/ml). Milk SOX activity has been detected in the stomach and proximal small intestine contents of suckling rats. Human and bovine SOX are relatively heat stable: 75% of the latter remains after treatment at 62.5°C for 30 rnin and 65% of the former remains after treatment at 607°C for 10 min. Neither remains after 62.5°C for 30 min.