Neutrophils are now considered central to the pathogenesis of most forms of acute lung injury. Neutrophils do not cause damage while suspended in the bloodstream; however, a release of cytotoxic agents occurs when neutrophils are adherent to endothelium, epithelium, or extracellular matrix proteins in the interstitium. Such neutrophil adherence is mediated predominantly through β2 integrins (CD11/CD18) on its surface. This study was undertaken to investigate whether the IκB/nuclear factor (NF)-κB cascade is involved in this β2 integrin-mediated activation of human neutrophils. β2 Integrin Mac-1 (CD11b/CD18) aggregation was induced by antibody cross-linking of the integrins on the cell surface. β2 Integrin aggregation induced interleukin-1β and tumor necrosis factor-α production, which suggests the activation of neutrophils by β2 integrin. IκBα was markedly degraded at 1 h, and NF-κB-DNA-binding activity markedly increased 2 h after β2 integrin aggregation, which activated IκB kinase activity at 1 h. β2 Integrin-induced cytokine production was suppressed by MG132 or SN50 pretreatment, which blocked the activation of NF-κB. These findings suggest that the activation of human neutrophils by β2 integrin aggregation is mediated through the activation of the IκB/NF-κB pathway.