Field and laboratory investigations have demonstrated that halogenated aromatic hydrocarbons (HAHs), which include PCBs and dioxin, are associated with developmental and population-level problems in fish-eating birds of the Great Lakes. Other studies have shown that perinatal exposure to HAHs causes thymic atrophy and suppresses T lymphocyte function in laboratory animals. Higher exposure suppresses antibody production and alters white blood cell (WBC) counts. This study investigated whether persistent contaminants alter immunocompetence in Great Lakes herring gulls (Larus argentatus) and Caspian terns (Sterna caspia). It also evaluated the use of various immunological tests as biomarkers for contaminant-associated health effects in wild birds. Masses of immune organs and WBC counts were assessed in herring gull chicks at 11 colonies and adults at 13 colonies, including two colonies outside the Great Lakes. T-cell- and antibody-mediated immune functions were assessed in chicks at five sites for each species. This ecoepidemiological study revealed a strong association between persistent contaminants and suppression of T-cell-mediated immunity. In herring gull chicks, thymus mass decreased as the activity of liver ethoxyresorufm-O-deethylase (EROD), an index of HAH-exposure and Ah-receptor activation, increased. In Caspian tern and herring gull chicks, the phytohemagglutinin skin test for T cell function showed a strong negative exposure-response relationship with organochlorines. There was no discernible association between contaminants and suppression of antibody-mediated immunity as measured by the sheep red blood cell antibody test and bursal mass. However, contaminant effects on bursal mass were confounded by fluke infections. Several WBC variables in both species were associated with contaminants, but the evidence was weaker than for effects on T-cell-mediated immunity. The identity of the particular organochlorine( s) responsible for alterations of T cell function and WBC counts could not be determined because concentrations of organochlorines were highly co-correlated in bird tissues. However, PCBs were the most likely cause because of their high concentrations and immunotoxic potential. Tests of immune function, WBC counts, and immune organ masses are useful biomarkers for assessing health effects, including those associated with contaminants, in wild birds.