Binding to estrogen and aryl hydrocarbon receptors (ER and AhR) is a possible mechanism for a compound to mediate endocrine-disrupting effects. In the present study, substances contained in sediment organic fractions from the Detroit River were screened for their ability to bind ER and/or AhR. Two in vitro bioassays were used: MELN cells, consisting of human breast cancer cells (MCF-7) stably transfected with the luciferase reporter gene, to assess the binding to ER, and cultured fish hepatocytes, PLHC-1, to measure the AhR-mediated induction of 7-ethoxyresorufin-O-deethylase (EROD). Chemical analyses of the sediment extracts were performed to examine relationships with the biological responses. Significant ER- and AhR-activities were mostly found in the most polar fraction (containing mostly PAHs). The five strongest estrogenic extracts were also the most efficient to induce EROD activity. The correlation between PAHs contamination level and biological responses suggests that the most polluted areas contain a mixture of both estrogenic and EROD-inducing compounds or, alternatively, compounds able to bind ER and AhR, such as PAHs, would be responsible for the observed activities.