Environmental salinity is a very important factor for aquatic organisms and any changes in salinity seriously affect physiological processes. In fish, studies on the effect of salinity have mainly looked at changes in osmoregulatory organs and their hormonal control, plasma parameters, energy metabolism, growth, etc. [1e3]. However, little is known about the changes in the fish immune system after salinity disturbance despite the fact that salinity is one of the most important environmental factors in the aquatic medium. The transfer from freshwater to seawater decreases rainbow trout (Salmo salar) specific-antibody titers [4], but in brown trout (Salmo trutta) it increases or has no effect on serum lysozyme activity, or leucocyte phagocytosis and cytotoxicity [5]. In the grouper fry (Epinephelus sp.) it has been demonstrated that specimens exposed to either hypoor hyper-osmotic shock increase their susceptibility to infectious pancreatic necrosis virus (IPNV) [6]. Gilthead seabream is the most important commercial fish for Mediterranean aquaculture. In some cases the fish farms are situated in waters with extreme salinity levels or which suffer salinity alterations (bays, natural ponds, places with freshwater inputs, etc.). This is why several studies have focused on the effects of salinity on some physiological responses, mainly osmoregulation and energy metabolism [7e14], as well as providing some information about the negative impact of several stressors on the immune system of this teleost species [15e18]. However, there are no data on the impact of salinity disturbance on the seabream immune system. This led us to carry out the present study.