The pathogenesis of Shigella flexneri infection centers on the ability of this organism to invade epithelial cells and initiate an intense inflammatory reaction. Because NF-κB is an important transcriptional regulator of genes involved in inflammation, we investigated the role of this transcription factor during S. flexneri infection of epithelial cells. Infection of HeLa cells with invasive S. flexneri induced NF-κB DNA-binding activity; noninvasive S. flexneri strains did not lead to this activation. The pathway leading to NF-κB activation by invasive S. flexneri involved the kinases, NF-κB-inducing kinase, IκB kinase-1, and IκB kinase-2. NF-κB activation was linked to inflammation, because invasive S. flexneri activated an IL-8 promoter-driven reporter gene, and the κB site within this promoter was indispensable for its induction. Microinjection of bacterial culture supernatants into HeLa cells suggested that LPS is responsible for NF-κB activation by S. flexneri infection. In conclusion, the eukaryotic transcription factor NF-κB was activated during S. flexneri infection of epithelial cells, which suggests a role for this transcriptional regulator in modulating the immune response during infection in vivo.