Cadmium is a non-essential element that can have severe toxic effect n aquatic organisms when it presents in excessive amount. It is necessary to understand its effects on the different blod parameters in edible fishes as common carp ( Cyprinus carpio). These common carps were obtained from a local fish farm in Tehran province, were acclimated in holding tank for one week . After dividing 30 apparently healthy fish (mean weight 700g) into two groups (control :Ι& test :Π) ,we transferred them to 1000 liter indoor fiberglass tanks supplied with static well water and with continues aeration .Test group was exposed to 30 ppb cadmium as CdCl2 (Merck Co.). Blood samples were obtained from 12 fish in each tank by using exsanguinations of caudal vein at the day 15 and 30 post exposure and the level of serum enzymes , LDH (lactate dehydro genase),ALT( alanine aminotransferase ), AST( aspartate aminotransferase ) and ALP ( alkaline phosphatase ), were measured by DGKC , IFCC , IFCC and DGKC methods respectively , Ependorff Analyzer based on company manual . The results revealed that level of LDH in test group (exposed to 30ppb Cd) significantly (p<0.05) increased in comparison with control group ( 430±69.3470) to(827.8±140.8215) after 30 days exposure .In test group LDH significantly (p< 0.05) were increased from (384±219.88) in the day 15 to (827.8±140.8215) in the day 30 . No significant changes were measured in the levels of other serum enzymes in test group in comparison with control one. This study showed that the level variation of enzyme serums (ALP, AST, ALT ) can not be used to specify sublethal cadmium toxicity in common carp . The exposure duration of sublethal concentration of cadmium was important parameter for increasing the level of LDH in common carp .
