Avian antimicrobial peptides, classified as beta-defensins, have been identified from bloods of chicken, turkey, and ostrich; epithelial cells of chicken and turkey; and king penguin stomach contents. beta-Defensins are a family of antimicrobial peptides characterized by six cysteine residues forming beta-defensin motifs that are also found in bovine, ovine, pig, and human. These peptides are active against a wide range of microorganisms including Grain-positive and Gram-negative bacteria, fungi, and yeast. Analysis Of evolutionary relationships of vertebrate beta-defensins showed that there might be a common ancestral gene between avian and other mammalian peptides. This ancient gene may have been passed down and evolved from species older than the oldest living birds, forming a beta-defensin-like precursor molecule. This review describes potential applications of these peptides in health care products. (C) 2004 Elsevier Inc. All rights reserved.