Abstract: Lysozyme activity of grass carp (Ctenopharingodon idella) weighing 850 g were ssessed following exposing fish to various concentrations of diazinon at 1, 2 and 4mg/L provided as a bath for 12 hours at 20±2?C. Compared with the control fish there was no significant difference in levels of lysozyme in tissues of spleen and kidney of experimental fish after one day post-exposing to the toxicant (p>0.05), whilst lysozyme contents in sera
of experimental fish were significantly lower than control fish (p<0.05). After 7 day-post-exposing fish to toxicant, the lysozyme contents in both spleen and kidney of fish exposed to 2 and 4 mg/L of toxicant were significantly higher than control fish (p<0.05). Also, compared with control group, no significant difference was observed in lysozyme content in sera and tissues of kidney and spleen of experimental fish 15, 30 and 45 days post-exposure (p>0.05). These results show that diazinon at sublethal concentrations stimulate some non-specific immune defence mechanisms of grass carp through enhancing of lysozyme content in heamatopoeitic tissues and blood serum. J.Vet.Res. 62,2:49-52,2007.