The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates immunosuppression induced by a variety of ubiquitous environmental pollutants, including polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and dioxins. Although the normal physiological role for the AhR in the absence of environmental chemicals is uncertain, recent studies suggest its contribution to cell growth and apoptosis. Because B cells seem to be directly affected by AhR ligands in animal models, it was postulated that the AhR is predominantly expressed in activated human B cells and that it may contribute to cell growth regulation. To begin to address these issues and to extend detailed analyses of AhR function to a human system, AhR expression in resting and activated human B cells was studied. In addition, the response of activated B cells to an environmental AhR ligand was investigated to provide insight into a possible physiological role for the AhR. Resting peripheral human B cells expressed little or no AhR. However, activation with CpG or CD40 ligand profoundly up-regulated AhR mRNA and protein. AhR nuclear translocation, constitutive DNA binding, and induction of an AhR-regulated gene, CYP1A1, in stimulated B cells in the absence of exogenous ligands suggested constitutive AhR activation. Cell division was not required for AhR up-regulation. Treatment of AhR-expressing B cells with a prototypic environmental AhR ligand, benzo[a]pyrene, significantly suppressed cell growth. These data help explain the sensitivity of B cells to environmental AhR ligands and strongly suggest that the AhR plays an important function within the human B cell compartment.