The effects of dietary $eta$-glucan administration on non-specific immune parameters in common carp, Cyprinus carpio, (1.0 g and 68.7 g of body weight) and flounder, Paralichthys olivcaces (12.1 g and 54.0 g of body weight) were evaluated. All fishes were fed an experimental diet supplemented with $eta$-glucan at $0.1\%$ per kg diet for 5 weeks. A week intermission with basal diet occurred between first 2 weeks and second 2 weeks of $eta$-glucan administration, The changes in the numbers of peripheral neutrophils and macrophages were counted under light microscopy and serum lysozyme activity was also analysed at a week of interval during the experiment. Phagocytic activities of leucocytes from the swimm bladder of carp and the peritonium of flounder were measured 5 weeks after feeding. The oral adminisration of $eta$-glucan induced significant reduction in mortality after an artificial challenge with $1 imes10^6$ cells of Aeromonas hydrophila in larger carp and $1 imes10^5$ cells of Edwardsiella tarda in larger flounder but did not in other groups. The numbers of peripheral macrophages and neutrophils, phagocytic acitivies of leucocytes, and the activity of serum lysozyme were greatly increased in the fish fed a $eta$-glucan supplemented diet. These suggest that $eta$-glucan administration by oral route can enhance leucocyte phagocytic activity, serum lysozymal activity, and survival rate against artificial infections depending on the infected fish size and challenged bacterial concentration.