Tryptophan (TRP) has been implicated in the mechanism of induction of microsomal aryl hydrocarbon hydroxylase (AHH) activity by UV light. UV irradiation of TRP resulted in formation of chloroform-soluble photoproducts that can specifically bind to the Ah receptor (AhR), an intracellular protein that mediates the induction of AHH activity by xenobiotics. Gel retardation analysis revealed that the TRP photoproducts, like other AHH inducers, complex to its specific DNA recognition site. Transient transfection experiments also demonstrated that the photoproducts could induce gene expression in an AhR-dependent manner. Thus, induction of AHH activity by UV light could be attributed, at least in part, to the formation of TRP photoproducts, which appear to act as AhR ligands.