The development of a selection procedure is described which utilizes the fact that certain polycyclic aromatic hydrocarbons are rendered highly cytotoxic when illuminated with near-ultraviolet light. Twenty polycyclic aromatic hydrocarbons were screened for phototoxicity and toxicity in the absence of light in the mouse hepatoma line Hepa1c1c7, which has high inducible aryl hydrocarbon hydroxylase (AHH) activity, and in AHH-deficient mutants derived from this line. In the assessment of phototoxicity, a period of time was allowed for the metabolism of these compounds prior to illumination. Benzo(g,h,i)perylene had the greatest phototoxicity in cells lacking AHH but was not toxic to cells possessing AHH either in the presence or absence of light. We show that, under conditions of our selection procedure, cells which possess AHH and thus are capable of metabolizing, and therefore of eliminating, benzo(g,h,i)perylene (as determined by the magnitude of the fluorescence of the compound in the cells) are resistant to subsequent exposure to near-ultraviolet light, while cells which lack AHH and are thus unable to eliminate the compound are killed by subsequent illumination. We show, furthermore, that cells possessing AHH can be selected from a majority population of cells lacking the enzyme by this procedure.