The possibility that NO2 artifactually converts polynuclear aromatic hydrocarbons to biologically active species in diesel particle sampling is explored. NO2 was injected into dilution air upstream of the exhaust mixing point at varvinq concentra-ting levels with both passenger car and truck driving simulations. Ames test specific activity and nitropyrene levels were seen to increase above base levels when the NO2 level exceeded 5 to 10 ppm. Extract responses to added NO2 in transient drivinq appeared to initially increase, then level off above about 20 ppm. It is suggested that some requestering of nitratable orqan-ics may be responsible. Reexposure of filtered particles to diluted gas phase diesel exhaust caused little increase in nitroaromatics or Ames activitv. It appears that NO2 levels below about 5 ppm are relatively safe from filter artifacts.