Alveolar macrophages (AM) located on the alveolar surface are directly exposed to air pollutants. We evaluated the effect of exposure to SO2 on the oxidative metabolism of AM and peripheral blood mononuclear cells (PBMNC) by measuring the spontaneous and stimulated reactive oxygen-intermediates (ROI) release. AM or PBMNC were placed on a polycarbonate membrane, which was in contact with the surface of a reservoir filled with RPMI 1640 allowing the cells to be supplied with nutrients by capillarity. For SO2 exposure times of 10, 20 and 30 minutes and concentrations of 2.5, 7.5 and 12.5 ppm were chosen. A 10-minute SO2-exposure up to 12.5 ppm induced a dose dependent maximal 3.6 fold increase of spontaneous ROI-production (r = 0.876; p < 0.005). A 30-minute exposure of 12.5 ppm SO2 exhibited a cytotoxic effect inducing the death of 62 +/- 9% of AM and caused a 63% decrease of ROI-release compared to 2.5 ppm SO2-exposure under identical conditions (r = -0.96; p < 0.005). These experiments demonstrate that AM and PBMNC are activated by SO2 and that concentrations in the range of 12.5 ppm SO2 are toxic and induce a decrease in ROI-release after 30 minutes exposure of these cells.