Pathogenic strains of Vibrio cholerae O1 elaborate a toxin-coregulated pilus, designated TCP, that is required for the bacteria to colonize the human intestine and cause disease. The possibility that antibodies directed against TCP might block colonization and thereby potentially prevent infection was investigated. The pilus was purified and polyclonal antiserum raised against it was shown to react preferentially with the 20.5-kDa major pilin subunit, TcpA. This antiserum inhibited attachment of the bacteria to epithelial cells in vitro. In a cholera animal model system, these pilus-specific antibodies efficiently protected infant mice from challenge with virulent V. cholerae strains of different serotypes and biotypes. Western immunoblot analysis of available killed, whole-cell vaccine preparations using TcpA-specific antibodies failed to detect pilin in either preparation. The results suggest that inclusion of TCP in cholera vaccines would provide a common antigen to induce immunity to the strains associated with human infection and potentially increase vaccine efficacy.