Teflon-coated glass fiber filters, used in studies of airborne particulate matter, were tested for mutagenic activity using the Salmonella/mammalian-microsome (Ames) assay. For each sample, eight blank filters were simultaneously extracted with dichloromethane (DCM), and the extract was concentrated by rotary evaporation and a nitrogen purge. Samples were solvent exchanged into dimethylsulfoxide (DMSO) for bioassay. Mutagenicity testing was performed using tester strain TA98, with and without metabolic activation. Preliminary findings have indicated that both direct- and indirect-extracting frameshift mutagens were present on the blank filters. Pre-extracting the filters with DCM reduced the activity associated with the filters; however, forced-air baking enhanced the mutagenic response. Additional analyses have suggested that the interference caused by filter mutagenic is inversely proportional to extraction efficiency and particle load.