Larval lysate (four-armed stage) exerts a lysozyme-like and antibacterial activity directed against Vibrio alginolyticus and other bacterial strains. This lysozyme activity is dependent on the pH and ionic strength of the reacting medium and sample. The other larval stages (six-armed, eight-armed and a stage approaching metamorphosis) possess protection against bacteria as well. The highest antibacterial activity is present in the six-armed stage. These antibacterial compounds avoid exploitation by bacteria. Paracentrotus lividus larvae used in our experiments were laboratory-reared from specimens collected in 1993 in Porto Cesareo (Lecce, Italy).