α1,3-Galactosyl antibodies (anti-Gal) are ubiquitous natural human serum and secretory polyclonal antibodies that bind to terminal galactose-α1,3- galactose (α-galactosyl) residues. Serum immunoglobulin G (IgG) anti-Gal can block alternative complement pathway-mediated lysis of representative gram- negative enteric bacteria that bind it to lipopolysaccharide α-galactosyl structures, thereby promoting survival of such bacteria in the nonimmune host. We wanted to know whether anti-Gal also could bind to the lipooligosaccharides (LOS) of Neisseria meningitidis. To our surprise, we found that serum and secretory anti-Gal bound to pili but not to LOS of certain strains. This suggested the presence of an immunogenic pilus carbohydrate epitope. Mild periodate oxidation of sodium dodecyl sulfate- polyacrylamide gel electrophoresis-separated outer membrane preparations from strains that bound anti-Gal followed by labeling of the neoaldehyde groups resulted in the labeling of hands that corresponded to pilin and LOS, confirming that pilin contains carbohydrate structures. A Bandeiraea simplicifolia lectin that also binds terminal α1,3-galactosyl residues also bound to pilin. Serum IgG, IgA, and IgM anti-Gal as well as colostral secretory IgA anti-Gal bound to pilin, as judged by immunoblotting, and to the pili of intact piliated organisms, as judged by immunoelectron microscopy. Total serum anti-Gal (IgG, IgA, and IgM) and purified serum IgA1 anti-Gal, but not its purified IgG isotype, blocked complement-mediated lysis of a piliated meningococcal strain that bound anti-Gal to its pili. Colostral anti-Gal secretory IgA blocked killing of the same strain. Thus, anti-Gal IgA may promote disease when it binds to the pili of N. meningitidis strains.