Peroxidase enzymes present in human colostrum, saliva, polymorphonuclear leukocytes, and bovine milk were compared with respect to their molecular exclusion chromatographic behavior and immunological cross-reactivity. Human milk peroxidase gave an elution profile similar to myeloperoxidase derived from blood polymorphonuclear leukocytes. Human salivary peroxidase reacted with an antibody directed against bovine lactoperoxidase, but with the same antibody preparation no reaction was detected either with human milk peroxidase or leukocyte myeloperoxidase. We conclude that the peroxidase enzyme in human milk is different from the human salivary and the bovine enzymes and is probably derived from milk leukocytes. © 1982.