Groups of female NMRI-mice inhaled nine weeks 12.4 or 81.8 microgram Pb/m3 24 h per week, while other groups inhaled 0.3 mg NO2 + 5 mg flame soot/m3 or 5 mg NO2 + 0.3 mg flame soot/m3 for 45 h/week. Five animals of each group were randomly selected in weekly intervals and bacterial elimination determined 5 hours after inhaling a Serratia marcescens-aerosol. Bacteria in lung sections were determined by means of the "sandwich-method", using an anti-Serratia-serum and a FITC-loaded antirabbit-gammaglobuline. Inhalation of leadchloride caused a time and dose-dependent deterioration of bacterial elimination, which showed to be statistically significant already after three days of treatment with 81.8 microgram Pb/m3. A time dependent function between bacterial elimination and exposure could not be shown under treatment of the mixed pollutants NO2 and flame soot, although the lung clearance was deteriorated especially in the group treated with 5 mg NO2 + 0.3 mg flame-soot/m3. The experiments give evidence that lead exhibits a cyto-toxic effect on alveolar macrophages while the combined pollutants NO2 and flame-soot exhibit their adverse effect on the mucociliary-system. Nitrogen dioxide is shown to be a more hazardous pollutant than flame-soot within the given combination.