As part of the Integrated Air Cancer Project (IACP), diluted emissions from wood stoves and automobiles were injected into a Teflon smog chamber and irradiated to simulate their photochemical transformation in the atmosphere. Changes in the chemical composition and physical properties of the gaseous and aerosol-bound complex mixtures were monitored throughout the experiments. The mutagenicity of the gas-phase components and of the aerosol-bound chemicals were both measured before and after irradiation. Comparisons of the mutagenic activity between the gas-phase and aerosol-bound chemicals have been estimated. When the mutagenicity is expressed in units of revertants per cubic meter of air, the gas-phase reaction products are found to be the most mutagenic. Similar results were obtained from preliminary experiments using an idling automobile as the pollutant source.