In 2001–2002, Great Black-backed Gull (Larus marinus) eggs from Lake Ontario had significantly higher concentrations of organochlorine contaminants than Herring Gull (L. argentatus) eggs. This study investigated whether higher contaminants in Great Black-backed Gulls affected immunocompetence, packed cell volume (PCV) and body condition in chicks, endocrine disruption and body condition in adults, and reproductive success. Variables were compared between species at two Lake Ontario sites (one in Ontario, Canada, and one in New York, USA), and with a control site in the Bay of Fundy, Canada. For Herring Gull chicks, body condition and total antibody titers to sheep red blood cells were greater at one contaminated site; PCV, phytohemagglutinin-P stimulation and reproductive output did not differ among sites. For Great Black-backed Gull chicks, PCV was significantly lower at one contaminated site; there were no other significant differences among sites. Between species, Herring Gull chicks had significantly higher antibody responses to sheep red blood cells at two sites and higher PCV at one site. Vitellogenin was not detected in the plasma of any adult males. Body condition of adult male Herring Gulls was significantly greater on Lake Ontario than in the Bay of Fundy control site. High contaminant concentrations in Great Black-backed Gull eggs from Lake Ontario were not associated with differences in phytohemagglutinin-P stimulation or body condition in chicks, nor with differences in reproductive output, compared with Herring Gulls, or with Great Black-backed Gulls at a control site.