An antibacterial peptide with 16 amino acid residues was found in plasma of the freshwater crayfish, Pacifastacus leniusculus. This peptide, designated astacidin 1, was purified by cation-exchange column chromatography and reverse-phase high performance liquid chromatography. Astacidin 1 has a broad range of antibacterial activity, and it inhibits growth of both Gram-positive and Gram-negative bacteria. The primary sequence of astacidin 1 was FKVQNQHGQVVKIFHH-COOH. The molecular mass was 1945.2 Da, and no carbohydrate-linked amino acid residues could be found by mass spectrometry. A synthetic astacidin 1 resulted in similar activity as the authentic astacidin 1 against Gram-positive bacteria, whereas it had less or no activity against Gram-negative bacteria. Three amino-terminal-truncated synthetic peptides were made; they all showed low activity, suggesting that the amino-terminal part of astacidin 1 contributes to the antibacterial activity. The structure of astacidin 1 based on the CD results showed that it has a beta-sheet structure in citric acid buffer at pH 4, 6, and 8. Cloning of astacidin 1 shows that it is the carboxyl-terminal part of crayfish hemocyanin and that astacidin 1 is produced by a proteolytic cleavage from hemocyanin under acidic conditions. The processing and release of astacidin 1 from hemocyanin is enhanced when crayfish are injected with lipopolysaccharide or glucan.