Chemiluminescent response of grass carp (Ctenopharyngodon idella) was assessed to determine the effect of various concentrations of diazinon on the phagocytosis of grass carp in order to evaluate the fish immunity after exposure to this toxic chemical. One hundred and twenty specimens with individual weight of 850±155g were exposed to various concentrations of diazinon at 1, 2 and 4mg/L provided as a bath for 12 hours at 20±2°C in Institute of the Caspian Sea Ecology, Sari in 2003. Maximal values of spontaneous chemiluminescent responses (SCL) were found in all experimental fish only on days 1 and 7 post-exposing to the toxicant compared to control one (p0.05). Generally, the level of chemiluminescent response activated by zymosan (CRAZ) at all concentrations of toxicant were significantly higher than control group after 1, 7, 15, 30 and 45 days post-exposure to the toxicant (p<0.05)
