The reactants and products in irradiated propylene/NOx mixtures were brought to a steady-state distribution in a Teflon smog chamber operated in a dynamic mode. The effluent from the chamber was then tested for total mutagenic activity by exposing Salmonella typhimurium strain TA100 to it. The data show an increased mutagenic activity for the products when compared with the reactants and controls. In addition, the mutagenic activity at long reaction times is substantially greater than at short reaction times. To examine a subset of the propylene/NOx photooxidation products, an exposure of strain TA100 to the products of the propylene/N2O5 dark reaction was conducted. Although a small mutagenic activity was observed for the mixture, a number of mutagenic organic nitrates were identified. The results for the irradiated propylene/NOx mixture were analyzed in terms of the mutagenic activities of the individual products. The major products (carbon monoxide, ozone, formaldehyde, acetaldehyde, nitric acid, and peroxyacetyl nitrate) account for no more than 20% of the observed mutagenic response, assuming additivity. (Copyright (c) 1985 American Chemical Society.)