An antimicrobial peptide, ostricacin-1, has been purified and characterized from ostrich leukocytes. The peptide has a mass of 4011 and contained 36 residues, including 3 intramolecular cystine disulfide bonds. Ostricacin-1 has a primary sequence homology to the beta -defensin family and was active at 6.7 mug ml(-1) against E. coli and Staphylocccus aureus in vitro.