In 2001, organochlorine contaminants in great black-backed gull (' Larus marinus') eggs from Lake Ontario were significantly higher than a control site (Bay of Fundy), and higher than in herring gull ('Larus argentatus') eggs from Lake Ontario. In great black-backed gull chicks, packed cell volume was significantly lower at one contaminated site. Response to PHA-P stimulation, antibodies to sheep red blood cells and body condition did not differ among sites. In herring gull chicks, response to PHA-P stimulation was significantly lower at one contaminated site than the control. Antibodies to sheep red blood cells, body condition and packed cell volume did not differ among sites. Antibodies to sheep red blood cells were lower in great black-backed gulls than herring gulls at one contaminated site and the control site, packed cell volume was lower in great black-backed gulls at one contaminated site. Vitellogenin was not detected in adult males of either species.