Ozone (O3), a major component of air pollution and a strong oxidizing agent, can lead to lung injury associated with edema, inflammation, and epithelial cell damage. The effects of O3on pulmonary immune cells have been studied in various in vivo and in vitro systems. We have shown previously that O3exposure of surfactant protein (SP)-A decreases its ability to modulate proinflammatory cytokine production by cells of monocyte/macrophage lineage (THP-1 cells). In this report, we exposed THP-1 cells and/or native SP-A obtained from bronchoalveolar lavage of patients with alveolar proteinosis to O3and studied cytokine production and NF-κB signaling. The results showed 1) exposure of THP-1 cells to O3significantly decreased their ability to express TNF-α in response to SP-A; TNF-α production, under these conditions, was still significantly higher than basal (unstimulated) levels in filtered air-exposed THP-1 cells; 2) exposure of both THP-1 cells and SP-A to O3did not result in any significant differences in TNF-α expression compared with basal levels; 3) O3exposure of SP-A resulted in a decreased ability of SP-A to activate the NF-κB pathway, as assessed by the lack of significant increase and decrease of the nuclear p65 subunit of NF-κB and cytoplasmic IκBα, respectively; and 4) O3exposure of THP-1 cells resulted in a decrease in SP-A-mediated THP-1 cell responsiveness, which did not seem to be mediated via the classic NF-κB pathway. These findings indicate that O3exposure may mediate its effect on macrophage function both directly and indirectly (via SP-A oxidation) and by involving different mechanisms.