Much is known about bovine lactoperoxidase but no data are available on its primary structure. In this work its main active fraction was isolated from cow's milk and sequenced using a conventional strategy. A clear similarity was found with human myeloperoxidase, eosinophil peroxidase and thyroperoxidase, the sequences of which were recently elucidated from those of their cDNAs and/or genes. The single peptide chain of bovine lactoperoxidase contains 61 2 amino acid residues, including 15 half-cystines and 4 or 5 potential N-glycosylation sites. The corresponding peptide segments of human myeloperoxidase, eosinophil peroxidase and thyroperoxidase display 55%, 54% and 45% identity with bovine lactoperoxidase, respectively, with 14 out of the 15 half-cystines present in each of the four enzymes being located in identical positions. The occurrence of an odd number of halfcystines in bovine lactoperoxidase supports the recent finding of a heme thiol released from this enzyme by a reducing agent, suggesting that the heme is bound to the peptide chain via a disulfide linkage, since the absence of free thiol in the enzyme was reported long ago Closely related heme peroxidases have been detected in most mammalian exocrine secretions. They are believed to protect mucosal surfaces from microorganisms by catalyzing the production of toxic oxidizing agents from HzOz, and halides or SCN-, a pseudohalide. Two of them, bovine lactoperoxidase and human salivary peroxidase, have been especially studied (Tenovuo, 1985). Bovine lactoperoxidase was highly purified and characterized in the early forties (Theorell and Akeson, 1943). Although lactoperoxidase is able to oxidize Br-, I-and SCN-, Reiter and coworkers (1964) demonstrated that oxidation of SCN-was involved in the mechanism of the lactoperoxidase-catalyzed antibacterial effect in bovine milk. Aune and Thomas (1977) and Hoogendoorn et al. (1977) independently concluded that hypothiocyanite (OSCN-) was the main antibacterial species produced from SCN-and H 2 0 z .