The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that binds and mediates responses to many halogenated aromatic compounds (HACs). Exposure to mixtures of HACs frequently results in nonadditive behavior in both in vivo and in vitro assays. One cause is antagonism, which results when two or more ligands compete for a limited supply of the AhR; one interacts agonistically to induce a strong response, and the other interacts unproductively, eliciting little or no response. This study involves the mechanism by which HACs induce CYP 1A1. Agonistic (e.g., TCDD) and unproductive (e.g., PCB 153) HACs behaved similarly through the stages of initial AhR binding and conversion of the initial AhR-ligand complex to the form that possesses increased affinity for the bound ligand. They diverged in the ability of the AhR-HAC complex to bind to a synthetic oligonucleotide containing the consensus dioxin response enhancer sequence, as studied by the gel retardation assay. Competition for the Ah receptor was used to explain antagonistic behavior between TCDD and other HACs in both the gel retardation assay and the downstream response of CYP 1A1 induction in primary rat hepatocytes.