The O-specific polysaccharide (O-SP) domain of Shigella LPS is both an essential virulence factor and a protective antigen for this genus. A critical level of serum IgG anti-O-SP was shown to confer immunity to shigellosis, likely by complement-mediated bacteriolysis of the inoculum. Conjugate Shigella O-SP vaccines were shown to be safe and immunogenic in children, and, in a preliminary study, Shigella sonnei vaccine was protective in young adults. Characteristic of shigellosis is bacterial invasion of intestinal cells. Incubation of shigellae with postimmunization but not preimmunization sera of children vaccinated with S. sonnei or Shigella flexneri 2a O-SP conjugate vaccines inhibited in a type-specific and dose-dependent manner in vitro invasion of intestinal epithelial cells (Caco-2) and the infection-associated increases in IL-1β and IL-8 mRNA and extracellular cytokine levels. Pretreatment of these sera or of Caco-2 cells with O-SP abrogated these effects also in a type-specific and dose-dependent manner. Confocal microscopy demonstrated antibody-specific inhibition of bacterial adhesion to HeLa cells. These protective effects were duplicated by IgG purified from these sera. These results suggest a dual role for IgG anti-O-SP. In addition to lysis of the inoculum in immune individuals, the newly synthesized IgG anti-O-SP in patients may terminate an established infection by inhibiting shigellae released from epithelial cells from invading new ones. A critical level of IgG anti-O-SP could, therefore, have a protective as well as a curative role in shigellosis.