As all life depends on the productivity of plants, it is important that the effect of exposure of plants to xenobiotics and their subsequent fate be evaluated. Although extensive information has been published on the effect and metabolism of nitro- polycyclic aromatic hydrocarbons (nitro-PAH) by animals, few studies on the metabolism of these xenobiotics by plants has been reported. This study was designed to determine whether a plant cell culture derived from alligator weed (Alternanthera philoxeroides) activates or detoxifies these PAH to more or less active mutagenic compounds. The mutagenic response elicited by Salmonella typhimurium TA98 in a coincubation assay with the plant cell culture was used. Significant detoxification of 1-nitropyrene (1-NP) and 1,3-, 1,6-, and 1,8- dinitropyrene (1,3-,1,6- and 1,8-DNP), all direct acting mutagens, was noted. The optimum cell concentration for the assay was 100 mg wet weight/ml. Denaturation of plant cell enzymes by autoclaving, arrested the enzymatic activity resulting in a 10% deactivation of the compounds. To determine whether uptake of these compounds into plant cells could be inhibited by the plant cell wall, intact and ruptured cells were used in the assay. Results showed that ruptured cells were more efficient in detoxifying each nitro-PAH than were whole cells. To assess the role of cytosol and cytosolic membrane-bound enzymes in the activation and detoxification of the PAH, the coincubation assay was performed with a S12 supernatant from sonicated cells. With this cellular preparation, an increase in the mutagenicity of 1,3- and 1,6- DNP was observed. In contrast to the results with DNP, the aromatic amine, 4-nitro-o-phenylenediamine was activated to mutagens by intact cells. From our results and others, it appears that plants can selectively activate /and or detoxify certain xenobiotic compounds.