An in-situ microbial mutagenesis assay system was presented which permitted entrapment of mutagenic airborne particles and vapors by impinging unfiltered ambient air into trapping medium containing bacterial tester cells. The trapping device consisted of a pump, impinger, and cyclone. Impinging air flow generated by the pump was about 3 liters per minute. In an attempt to detect the mutagenic activity of air pollutants with this system, a study was made of cigarette smoke, the condensates of which are known to be mutagenic. A burning cigarette was placed on a cigarette holder directly connected to the trapping system. Smoke was pulled by the pump into the medium containing the in-vitro microsomal activation system. In a second experiment, smoke pulled by the pump was released into the air inside a closed chamber. Five minutes after five cigarettes were burned, the smoke in the air, representing side stream smoke, was impinged into the trapping medium. Positive time related responses were noted in all experiments. The frequency of reversion and the number of revertants per plate was similar among the four different cigarette brands studied, indicating that mutagenic activity of cigarette smoke was independent of tar content. The side stream smoke study indicated that the mutagenic activity at a low concentration of cigarette smoke in ambient air was still detectable by this assay system. The authors recommend the system as simple, economical, and sensitive for detecting or monitoring ambient mutagenic substances in the workplace or other defined environments.