A novel big defensin was isolated and characterized from the plasma of Ruditapes philippinesis. It was purified to homogeneity by means of precipitation with (NH(4))(2)SO(4), gel-exclusion chromatography, two kinds of cation-exchange chromatography and named RPD-1. Its relative molecular mass was 24.8 kD by means of SDS-PAGE. By means of ABI437 amino acid sequence analyser, its 11 NH(2)-terminal amino acid sequence is AVPDVAFNAYG. Two databank systems (NCBI and EBI/EMBL) was indexed, no sequence with homology to RPD-1 was found. Furthermore, it exhibited strong inhibition on the growth of Gram-negative and -positive bacteria. Minimal inhibitory concentration (MIC) of RPD-1 against Staphylococcus aureus, Bacillus subtilis, Micrococcus tetragenus, Escherichia coli, Vibrio parahaemolyticus, Vibrio anguillarum was 9.6 mg/L, 76.8 mg/L, 38.4 mg/L, 76.8 mg/L, 19.2 mg/L, 19.2 mg/L respectively.