This study hypothesizes that sewage treatment lagoons of Iowa towns contain levels of estrogenic substances that are sufficient to cause endocrine disruption in fish when they discharge to streams. Effluents of municipal wastewater treatment facilities contain alkylphenol polyethoxylates (APEs), which degrade to products that act as estrogen mimics, as well as ethynylestradiol. In this study, blood samples of caged fish held in lagoons will be examined for the presence of a specific blood protein, vitellogenin (VTG), which is normally produced by females. VTG is a biomarker of endocrine disruption when it is found in elevated concentration in female fish and present in the blood of male fish. Quantitative techniques will be developed to assess the level of VTG being produced by the fish.