Reproductive impairment of double-crested cormorants (Phalacrocorax auritis) and Caspian terns Hydroprogne caspia) has recently been observed in the Great Lakes of North America. Planarchlorinated hydrocarbons (PCHs), which include polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDS), and polychlorinated dibenzofurans (PCDFs), are believed to be at least partially responsible for the increased incidence of teratogenesis and embryonic mortality. The objective of the study was to assess the potency of the PCH mixtures in egg extracts from these colonial waterbirds. The evidence is strong for at least a partial role of PCHs as causal agents in the reproductive impairment of fish-eating waterbirds from the Great Lakes of North America. (Copyright (c) 1990 Elsevier Science Publishers Ltd, England.)