Immunoglobulin G (IgG) fractions prepared from sera of Shigella dysenteriae 1 infected patients during the acute and convalescent phases of illness were found to be effective inhibitors of adhesion of S. dysenteriae cells to guinea pig colonic epithelial cells in vitro. The adhesion could also be inhibited by whole sera, their IgG fractions, and Fab fractions prepared from sera of rabbits immunized with whole bacteria, the outer membrane (OM) and lipopolysaccharide. The adhesive capability was best inhibited by the Fab fragment of antisera of rabbits immunized with whole bacteria.