The present project includes one pilot study of wild adult glaucous gull (Larus hyperboreus) and one experimental study of glaucous gull chicks raised in captivity. The pilot study of adult gulls gave us enough blood and tissue samples to develop the methods needed for immune system analysis in the laboratory experiment. In the experimental study a total of 39 glaucous gull chicks were hatched and raised in captivity in Svalbard, Norway. The chicks were divided into two groups. One experimental group (20 chicks) was given food that mimicked the “natural” food found in the marine environment. The control group (19 chicks) was given “clean” food. After 56 days the chicks were sacrificed in order to collect samples for analyses of organochlorines (OCs) and immunocompetence measurements. The experimental group had 2.8, 3.9, 5.0, and 6.1 time’s higher concentrations of HCB, Oxychlordane, ?DDT, and ?PCB, respectively, compared to the control group at day 56. All chicks used in the experiment were immunised with various vaccines and sera in order to test their ability to respond against foreign antigens. The experimental chicks produced low levels of virus neutralising antibodies when tested against the herpes virus and reovirus. They produced higher levels of neutralising antibodies when tested to tetanus toxoid. There was, however, no difference between the experimental groups with regard to the mean antibody titres. The chicks in both groups also responded to the influenza virus by increasing the production of specific antibodies. However, the mean antibody titre in the exposed group was significantly lower than in the control group. The mitogen-induced response of blood lymphocytes to PHA and LPS was significantly higher in the exposed group compared to the control group. The specific response of blood lymphocytes to Con A, PWM, KLH, TET, and PPD was higher in the exposed group compared to the control group. However, do to high variance in the exposed group there was no significant difference between groups with regard to the lymphocyte response to these mitogens. The results from the present study indicate a toxic effect of OCs on the glaucous gull chicks, which induced a systematic activation of the immune system. Further work on data will be performed.