Injection of live nonpathogenic bacteria, Esherichia coli K 12 D 31 into pupae of the Chinese oak silkworm, Antheraea pernyi, induced antibacterial peptides in the hemolymph. The major antibacterial peptide was purified with CM-Sepharose cation-exchange chromatography, reverse phase chromatography on FPLC system and C 18 column reverse phase chromatography on HPLC system. The purified peptide had a molecular mass of about 2 400 as determined by SDS-PAGE and its amino acid composition was shown to be rich in lysine , alanine and glycine, while lacking in methionine , cysteine , tyrosine and histidine. The molar content of lysine, alanine and glycine was 18 6%,15 5%,13 6% respectively. Inhibitory spectrum showed that it was potent against several Gram-negative and Gram-positive bacteria.