An investigation was made on haematological parameters and enzyme systems in Penaeus monodon during white spot syndrome virus (WSSV) infection. White spot syndrome was developed by injecting WSSV inoculum into the prawn. Animals were succumbed to death within a short period of 72 -84 h after injecting the inoculum. Haematological parameters like total haemocyte count,(THC), clotting ability of haemolymph and levels of glucose, protein, creatinine, albumin/ globulin ratio were decreased during viral infection. Virus attack affected the defense system of prawn, which was evident from the decreased activity of phenol oxidase enzyme. Failure of antioxidant system, marker enzymes, enzymes in carbohydrate metabolism were also noted.Among the different natural and artificial immunostimulants tried in the feed for prawn, the performances of thulasi and cyclopez were better than others.in fighting WSSV infection.