Aryl hydrocarbon hydroxylase (AHH) levels were measured in the livers of herring gull (Larus argentatus) embryos to determine if monooxygenase activity in this species could be used as a “bioeffects” monitor of environmental contamination. Herring gull eggs were collected in 1981 from five locations on the Great Lakes Basin and from one clean colony on the Atlantic coast. Microsomal protein determinations and AHH assays were conducted on livers from the embryos at 20 and 25 d of incubation. Organochlorines (DDE, DDT, tetrachlorobenzene, pentachlorobenzene, hexachlorobenzene, heptachlor epoxide, chlordane, dieldrin, mirex, photomirex, polychlorinated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin) were also measured in egg homogenates from each colony. The 25-d embryos were found to be the most suitable for AHH analysis, since they had a higher baseline level and a greater response than did the 20-d embryos. Specimens from two of the Great Lakes colonies had significantly higher AHH activities than was found for the Atlantic coast colony. These levels of AHH activity closely paralleled the concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin present in eggs gathered from the colonies in 1980. Further work regarding monooxygenase levels in herring gull embryos is discussed.