In present study,the enzymatic activities of the peroxidase(POD),lysozyme(LSZ),superoxide dismutase(SOD)and phenoloxidase(PO)have been monitored by use of biochemical techniques.Meanwhile,the transmission electron microscopy(TEM)method has been applied to compare the morphological changes in hepatopancreas of juvenile redclaw crayfish, Cherax quadricarinatus, under two experimental conditions:one manual injection of immune polysaccharide; two White Spot Syndrome Virus(WSSV)stress exposure.Four experimental groups have been set up in this study:control group［with PBS(pH 7.4)injection］; experimental group Ⅰ(with WSSV injection); experimental group Ⅱ(with immune polysaccharides injection); experimental group Ⅲ(with immune polysaccharides injection first and with WSSV injection after 48 hours).The results of biochemical studies showed that:in comparison with the control group,the activities of POD,PO,LSZ in experimental group Ⅰ were decreasing significantly(P<0.01)in 72 hours.In contrast,the activity of SOD in experimental group Ⅰ was increasing first and then decreasing,the enzyme activity was significantly lower(P<0.01)in 72 hours; in the experimental group Ⅱ,the enzymatic activities of POD,PO,LSZ,SOD were all increasing first and then decreasing.Their enzymatic activities were all higher than the control group(P<0.05).In detail,the SOD and LSZ activity reached the threshold after 24 hours immune polysaccharide injection,the POD had the highest activity in 48 hours,the maximum activity of PO reached after 12 hours injection; experimental group Ⅲ,the enzymatic activities of POD,PO,LSZ,SOD were all higher than experimental group Ⅰ(P<0.05)after 72 hours exposure.However,in comparison with the control group,the SOD,POD,LSZ activities were decreasing,with only PO enzymatic activity increasing first and then decreasing,four enzymes,activities were all decreasing significantly(P<0.01)in 72 hours in contrast with the control group.Four enzymatic activities in experimental group Ⅱ is greater than control group and enzyme activity in experimental group Ⅲ is greater than experimental group Ⅰ and this proves a direct evidence that the immune polysaccaride injection can improve the resistance of juvenile prawn to WSSV and be able to actively suppress the infection with WSSV.Moreover,the morphological study in hepatopancreas of C.quadricarinatus showed that:in control group,the hepatopancreas is composed of liver-tubules,the liver-tubules is made up of simple columnar epithelium cells and basement layers,with the simple columnar epithelial cells the neat microvilli on the surface and the liver-tubules epithelial cells structurally-complete.In experimental group Ⅰ,the mitochondria were seriously harmed and had abnormality;the karyotheca disassembly,substance external flow in nucleolus and cell nucleus become torn,at the same time the cell nucleus had the chromosome heterochromatin phenomenon.In addition,more serious of the rough endoplasmic reticulum becoming split liminated structure and ribosome is to reduce in experimental group Ⅰ; In experimental group Ⅱ,the cell nucleus chromatin uniform distribution on the rough endoplasmic reticulum(RER)ribosome showed an increase; In experimental group Ⅲ,RER become tumescence and few split.Meanwhile,the number of vesica increased,and the vacuole were also found increased.Certain portion of mitochondria turned into abnormality,regular ellipse or length structure mitochondria develop into irregular.Our results indicate that WSSV stress can damage the ultrastructure of hepatopancreas in juvenile C.quadricarinatus, and further influence the normal physiological function of C.quadricarinatus.Present study by biochemical techniques and TEM method further shows that the immune polysaccharides can promote the resistance of juvenile prawn to the WSSV infection.