Bay of Marennes-Oleron is an important French area of reproduction for sea bass, Dicentrarchus labrax. This area is often subjected to many pollutions due to anthropic activities, including pesticides and polycyclic aromatic hydrocarbons (PAH) which cause adverse effects in aquatic organisms. The harmful effects of such contaminants on fish immune system leading to a decreased resistance against pathogens begin to be described, but remain poorly known. In this context, influence of micropolluants, like PAH, on the defense mechanisms developed by sea bass was tested. Fish were exposed five days to a soluble fraction of PAH with environmental concentration (760 ng/L). At the end of the contamination, fish were transferred to non contaminated sea water and a kinetic study of blood immune parameters was assessed during one month. The effects of HAP were compared to those observed in a control group kept in non contaminated sea water. Leucocytes viability and activities were analyzed by flow cytometry. Plasmatic alternative pathway of complement activity and lysozyme concentration were performed. Dosage of PAH in fish tissues was carried out. Just at the end of the contamination, a significant increase of the cellular mortality was observed in contaminated fish compared to control fish. No difference neither on forward and side scatter profiles of blood cells population nor on phagocithic activity was found between the two groups whenever they were sampled. In contaminated fish, from day 3 to day 9, a significant higher haemolytic activity of complement, was noted. These results will be discussed.