Targeted ablation of the surfactant protein D (SP-D) gene caused progressive pulmonary emphysema associated with pulmonary infiltration by foamy alveolar macrophages (AMs), increased hydrogen peroxide production, and matrix metalloproteinase (MMP)-2,-9, and-12 expression. In the present study, the mechanisms by which SP-D influences macrophage MMP activity were assessed in AMs from SP-D−/− mice. Tissue lipid peroxides and reactive carbonyls were increased in lungs of SP-D−/− mice, indicating oxidative stress. Immunohistochemical staining of AMs from SP-D−/− mice demonstrated that NF-κB was highly expressed and translocated to the nucleus. Increased NF-κB binding was detected by EMSA in nuclear extracts of AMs isolated from SP-D−/− mice. Antioxidants N-acetylcysteine and pyrrolidine dithiocarbamate inhibited MMP production by AMs from SP-D−/− mice. To assess whether increased oxidant production influenced NF-κB activation and production of MMP-2 and-9, AMs from SP-D−/− mice were treated with the NADPH oxidase inhibitors diphenylene iodonium chloride and apocynin. Inhibition of NADPH oxidase suppressed NF-κB binding by nuclear extracts and decreased production of MMP-2 and 9 in AMs from SP-D−/− mice. SN-50, a synthetic NF-κB-inhibitory peptide, decreased MMP production by AMs from SP-D−/− mice. Oxidant production and reactive oxygen species were increased in lungs of SP-D−/− mice, in turn activating NF-κB and MMP expression. SP-D plays an unexpected inhibitory role in the regulation of NF-κB in AMs.