Cadmium is a problem of magnitude and ecological significance due to high toxicity and its ability to be accumulated in living organisms such as aquatic animals. Short time exposure to this heavy metal cause changes in several blood parameters and serious tissue damages in fish. Despite popular to the contrary, fish haematology is a valuable tool in disease diagnosis and prognostication ( Stoskopf M.K. 1993). Common carp were obtained from a local fish farm in Tehran province, and were acclimated in holding tank for one week. After dividing 30 apparently healthy fish (mean weight 700gr) in two groups (control and test) , they were transferred to 1000 liter indoor fiberglass tanks supplied with static well water and with continues aeration. Test group was exposed to30 ppb cadmium as Cd Cl2 ( Merck Co.) . Blood samples were obtained from 12 fish in each tank with triplicate by using exsanguinations of caudal vein at the day 15 and 30 post exposure and selected haematological indices such as: WBC, RBC, Differential count, haematocrite, globulin concentration s and the albumin /globulin ratio measured. The results revealed that White Blood Cell (WBC: ×10 4 ) count in cadmium group significantly (p≤0.05) decreased from 3.975 ± 1.326 in control fish to 2.52 ± 0.3033 0n day 15- post exposure in comparison with the day 15 post exposure. Also, the WBC count significantly (p≤0.05) decreased (1.7 ± 0.3317) on day 30 post exposure in comparison with the day 15 post exposure. No significant changes observed in the levels of another selected indices. It is concluded that 30 ppb of cadmium during sublethal exposure (15 & 30 day exposure) deacreased the level of WBC in common carp. So, the exposure duration was important for decreasing the level of WBC in common carp.
