Acrolein is an environmental pollutant that is known to suppress respiratory host defense against infections; however, the mechanism of the decrease in host defense is not yet clear. We have previously reported that acrolein inhibited endotoxin-induced cytokine release and induced apoptosis in human alveolar macrophages, suggesting that the inhibition of cytokine release and/or cytotoxicity to alveolar macrophages may, in part, be responsible for acrolein-induced immunosuppression in the lung. Because nuclear factor-B (NF-B) is an important transcription factor for a number of cytokine genes and is also an important regulator of apoptosis, the effect of acrolein on NF-B activity was examined by electrophoresis mobility shift assay. Acrolein caused a dose-dependent inhibition of endotoxin-induced NF-B activation as well as an inhibition of basal level NF-B activity. Because IB is a principal regulator of NF-B activity in the nucleus, changes in IB were determined by Western blotting. Acrolein-inhibited IB phosphorylation leads to an increase in cellular IB levels preventing NF-B nuclear translocation and is likely the mechanism of acrolein-induced inhibition of NF-B activity. The role of basal level NF-B in acrolein-induced apoptosis was also examined. An NF-B inhibitor (MG-132) also induced apoptosis in human alveolar macrophages, suggesting that a certain basal level NF-B activity may be required for macrophage cell survival. Taken together, our results suggest that the acrolein-inhibited endotoxin-induced NF-B activation decreased the basal level NF-B activity, which may be responsible for the inhibition of cytokine release and the induction of apoptosis in human alveolar macrophages.