allicin, sea bream, bactericidal activity, Cortisol, histopathology. We studied the effects of garlic (allicin) which used as immunostimulant on some physiological, biochemical parameters, survival rate, histopathological studies and bacteriological characteristics of sea bream. Fish (50 ± 5 g/fish) were assigned to 3 treatments, with three replicates each. Treatment groups had a different level of allicin (Garlen ® ), (1 and 2 gm. /kg feed) added to their diets; the control group diet was free

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... from garlic. Diets also contained 25% crude protein (CP) and were administered at a rate of 3% live body weight twice daily for 8 weeks. At the end of 7 th week ten fish from each treatment group and from the control were artificially infected by intra peritoneal injection with 0.2 ml of culture suspension of pathogenic Vibrio alginolyticus previously adjusted to 10 4 .Results showed that the differential leucocytic count, phagocytic activity, phagocytic index, serum lysozomal and bactericidal activity increased significantly with increasing levels of allicin. Plasma Cortisol level decreased significantly with increasing levels of allicin. Total protein and globulin were also increased significantly in the treated groups than the control group. All bacterial counts decreased significantly in treated groups than the control in all weeks. Treated groups had lower mortality rate than the control group during the challenge test. Histopathological studies of control group revealed congestion in blood vessel of gills, liver, spleen and kidney with, hyperplasia of epithelial cells between secondary lamellae led to fusion, edema and lifting of the lamellar epithelium of secondary lamellae in gills, vacuolar degeneration of hepatocytes, with infiltration by monocytes in liver, dilation of blood vessel, severe depletion of white bulb ,edema, and hyperactivation of melanomacrophage center in spleen ,necrosis of renal tubules epithelium, and inter renal haemopoietic tissues with atrophied glomerular tuft and widening of bowman capsules in kidney. Moderate to minimal histopathological changes appeared in all organs in groups received (Garlen ® ) according to its dose.