The BP8 variant of the 5L rat hepatoma cell line is completely devoid of aryl hydrocarbon receptor (AHR) and is a useful model to examine AHR function. Previous studies showed that BP8 cells, when transfected with mouse AHR, exhibit induction of a plasmid-based reporter even in the absence of exogenous ligands. We transfected BP8 cells with full-length human AHR and found that presence of the AHR alone was sufficient to induce substantial CYP1A1 and CYP1B1 mRNA without any exogenous AHR ligand. An AHR antagonist, 3.4-dimethoxyflavone, inhibited CYP1A1 and CYP1B1 expression in a dose-dependent manner. When we transfected BP8 cells with a mutated human AHR that is defective in ligand binding, expression of CYP1A1 and CYP1B1 was diminished but not abolished. Inhibition by the AHR antagonist along with the diminished response to the mutated AHR indicates that BP8 cells contain some agent that acts as an agonist ligand for the AHR. (C) 2004 Elsevier Inc. All rights reserved.