Rabbits were exposed (2h/d) to atmospheres consisting of 0.5 mglrn3(0.3 μm) H2SOAplus NO2at either 0.3 (low) or 1 ppm (high). Animals were sacrificed 24 h after 2, 6, or 13 exposures, and cells were recovered from the lungs by bronchopulmonary lavage. Exposure to high NO2with acid resulted in an increase in neutrophils at all time points and an increase in phagocytic capacity of macrophages after two or six exposures. On the other hand, exposure to the low NO2with acid resulted in depressed phagocytic capacity and mobility. The results were compared with those for NO2or H2SO4given alone.