The aromatic hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the biologic and toxicologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related chemicals. Here we utilized two AhR-dependent bioassay systems as screening tools to identify novel AhR agonists and to detect the presence of AhR agonists in sample extracts. These assays measure the ability of a chemical to activate AhR DNA binding in vitro (GRAB bioassay) or AhR-dependent (luciferase) gene expression in cultured cells (CALUX bioassay). Known AhR agonists (halogenated and nonhalogenated aromatic hydrocarbons) were positive in both assays, whereas the AhR antagonist alpha-naphthoflavone exhibited agonist activity only in the GRAB assay. In vitro GRAB analysis has identified several imidazoline receptor ligands and beta-carbolines as AhR agonists and also revealed the presence of AhR agonist activity in crude DMSO extracts of commercial newspapers. In contrast to their positive activity in the GRAB assay, the majority of these chemicals/extracts were only weakly active or inactive in the cell-based CALUX assay. Our results not only reveal that the ability of a chemical to activate the AhR in vitro does not necessarily correlate with its ability to induce gene expression in intact cells, but the high level of false positives obtained with the GRAB assay clearly demonstrates its inability to accurately identify AhR agonists or agonist activity. Screening of unknown chemicals, chemical classes, and samples for AhR agonist activity will require the use of intact cell bioassays.