An in-situ microbial mutagenicity test system for workplace airborne mutagen monitoring was described. The system involved trapping the mutagens by impingement followed by mutagenicity testing using the Salmonella histidine reversion and arabinose resistant tests. The trapping device consisted of a pump, an impinger, and a cyclone; the system drew test air into medium containing test bacteria. The tester strains used were the Salmonella-typhimurium testers that were resistant to streptomycin and 8-azaguanine, (TA-98W) and (TA- 100W), and the arabinose resistant tester, (SV-50W). These strains had comparable mutagenic sensitivities to 2-aminoanthracene and ICR- 191 as their parent strains. The microbial system with the new tester strains was tested using vapors of methyl-methanesulfonate (66273), ethyl-methanesulfonate (62500), and dimethylnitrosamine (62759). Mutagenicity increased as impingement time increased for all compounds and all three strains. The impingement method yielded a 62 to 77 percent recovery of methyl-methanesulfonate, and particle trapping properties were adequate as determined by tests with suspended silica particles coated with 2,4,7-trinitro-9-fluorenone (129793). Promutagen detection was accomplished by the incorporation of metabolic activation into the trapping medium through inclusion of S9 mix and tester cells in dialysis tubing. The authors conclude that this system could provide a simple and useful mutagenic monitor for workplace applications.