In order to determine the effects of low concentration of cadmium in some parameters that involved in fish innate immune system at the low temperature seasons, lysozyme activity, total white blood cell count and phagocytic activity in common carp (Cyprinus carpio) were estimated after 15 and 30 days exposure to low concentration of cadmium ( 30µg/l ). Forty eight common carp were obtained from a local fish farm in Iran, randomly distributed in two 1000 L fiberglass tanks. The fish were kept in condition similar to wintering pond conditions. All tanks were supplied with well water (hardness 320.6 mg/l as CaCO3, dissolved oxygen 8 mg L-1, pH 7, temperature 9 ºC ± 3), with continuous aeration and without water flow. The fish were starved during the whole trial. Fish in group I were kept in cadmium free water while group II exposed to 30µg L-1 (30ppb) cadmium as cadmium chloride for 30 days. Water quality was measured two times weekly throughout the exposure. At the days 15 and 30 post exposure, the immune response was monitored using haematological and serological assays. The results revealed that low concentration of cadmium significantly (p≤0.05) decreased the serum lysozyme content in common carp serum after 30 days. However, 15 days following exposure, the level of lysozyme in group II was similar to those recorded in control fish. At the day 15 th , a significant (p≤0.05) decrease in total leukocyte count and phagocytic index with considerable increase in monocyte took place in compare with control. thirty days post exposure white blood cell count in group II was significantly lower than it was in the day 15 th (p≤0.05). It seems decrease in total leukocyte count is depending on exposure time.
