Lysozyme is a widely distributed hydrolase which likely plays an important role in bio-defense systems. In the current study, we constructed a baculovirus expression system for the c-type lysozyme cDNA of Japanese flounder (Paralichthys olivaceus) and evaluated the activity of the recombinant protein. This activity was estimated, by turbidimetric assay, to be 7.7U/mg, a value five times higher than that of hen egg white (HEW) c-type lysozyme examined under the same conditions. The optimum pH and temperature for the lytic activity of the Japanese flounder c-type recombinant lysozyme were found to be 5.0-6.5 and 40 degrees C, respectively. Two groupings for fish lysozyme activity are proposed; the first has an optimum pH of approximately 6.0 and the second an optimum pH of above 8.0. Flounder c-type lysozyme was found to possess little lytic activities against Edwardsiella tarda and Streptococcus sp. This latter characteristic may help explain the fact that these two bacterial species are responsible for significant disease problems in cultured Japanese flounder.