The level of lysozyme in serum of common carp (Cyprinus carpio) was estimated after 15 and 30 days exposure to sub-lethal concentration of cadmium (30µg/l). Common carp were obtained from a local fish farm ( Amin abad , Tehran , Iran ) were Acclimated in holding tank for 1 week , then 50 apparently healthy, fish , mean weight 700g were randomly divided in to two groups ( I,II ) 1000 - l indoor fiberglass tanks supplied with well water [ hardness 320.6 mg/l as CaCo3, dissolved oxygen 8 mg/l , pH 7 , temperature 16 ºC ± 2 ], with continuous aeration and without water flow . Group I was control , group II exposed to 30µg/l cadmium as cadmium chloride. Water quality was measured two times weekly throughout the exposure .Serum samples from 12 fish in each tank were obtained at the days 15 and 30 post exposure and lysozyme level in serum were measured turbidimetrically. The results revealed that sub-lethal concentration of cadmium significantly (p≤0.05) decreased the lysozyme content in common serum after 30 days. How ever, 15 days following exposure , the level of lysozyme in group II was similar to those recorded in control fish .
