“…Many immunostimulants from di#erent sources have been studied in fish, including chemical agents, bacterial components, nutritional factors or polysaccharides. Among the polysaccharides, -glucans have been most studied in fish (Yano et al, 1989;Brattgjerd et al, 1994;Robertsen et al, 1994;Samuel et al, 1996), although lentinan, schizophyllan, chitin and chitosan have also been tested (Sakai et al, 1992;Ho#man et al, 1997). Chitin is a polysaccharide and is the main component of insect and crustacean exoskeletons, and the cell walls of certain fungi.…”