Irradiated mixtures of toluene/NOx/H2O/air were brought to a steady-state distribution of reactants and products in a 22.7 cu. m. flow-mode smog chamber, and the effluent was tested for mutagenic activity by exposing Salmonella typhimurum strains TA100 and TA98 to it. Two different product distributions were examined, with that corresponding to a longer extent of reaction yielding a greater mutagenic response. Vapor-phase concentration/time profiles for several aldehydes, cresols, peroxyacetylnitrate, toluene, nitrogen oxides, ozone, and particulate matter were obtained for the reaction chamber and exposure chamber air masses. Calculations were made as to the total quantity of each product species that deposited into the biotesting medium. The information was used to speculate on what may have caused the mutagenic response.