In a 22.7 cu.m. flow mode smog chamber, 5.4 ppm proylene was allowed to react with 0.9 ppm ozone either in the presence or absence of 0.2 ppm nitrogen dioxide. The steady-state reactant and product distribution was then tested for total mutagenic activity by exposing Salmonella typhimurium strain TA100 to the gasphase chamber effluent. The total product dosage in the test plates was varied by exposing them for 0, 5, 10, 15, and 20 h. Salmonella typhimurium survivor levels were obtained at each length of exposure. The number of revertants/plate increased at a rate of approx. 4-5 per hour, while the survivor level decreased throughout the exposure. Most of the total mutagenic activity can be accounted for by the presence of formaldehyde. This work clearly demonstrates that a gas-phase exposure of Salmonella typhimurium can be effectively employed as an alternative to the standard plate incorporation test for volatile species and for complex gas phase mixtures.