A novel toad lysozyme (named BA-lysozyme) was purified from skin secretions of Bufo andrewsi by a three-step chromatography procedure. BA-lysozyme is a single chain protein and the apparent molecular weight is about 15 kDa as judged by SDS-PAGE. The specific lytic activity against Micrococcus lysodeikticus of BA-lysozyme is 2.7 x 10(5) units/mg, indicating that it is a potent lysozyme. It displayed potent bactericidal activity against Staphylococcus aureus and Escherichia coli with minimal inhibitory concentrations (MIC) of 1 and 8 microM, respectively. The deduced primary structure of BA-lysozyme from cloned cDNA was confirmed by N-terminal sequencing and peptide mass fingerprinting. Its amino acid sequence shares 56.5% identity with that of chicken egg-white lysozyme. Phylogenetic analysis indicates that B. andrewsi lysozyme is closely related to that of turtle. This is the first report on the isolation and primary structure determination of amphibian lysozyme.