This report describes studies designed to assess the immunomodulatory effects associated with the consumption of coho salmon containing halogenated aromatic hydrocarbons (HAHs) and other compounds naturally bioaccumulated from Lake Ontario. Diets containing 33% coho salmon from Lake Ontario or the Pacific Ocean were fed to juvenile C57Bl/6 mice for 2-4 mo. Following 60 d, the mice that consumed Lake Ontario salmon had reduced IgM, IgG, and IgA plaque-forming cell responses to sheep erythrocytes. No changes were observed in total numbers of spleen lymphocytes, total T-lymphocytes or T-lymphocyte subsets as determined by flow cytometry. Cellular immunity, assessed by the cytotoxic T-lymphocyte response to allogeneic tumor target cells, was not altered following dietary exposure to Lake Ontario coho salmon for 4 mo. The observed humoral immunomodulation correlated with elevated PCB levels in the Lake Ontario salmon diets. The levels of pollutants such as mercury, tin compounds and other metals, PCDDs, and PCDFs were not examined.