A surface somatic antigen of some enterobacterial species, especially S. typhi. Its occurrence in freshly isolated strains of this organism may nullify agglutination by ‘O’ and ‘H’ antisera. Boiling the suspension destroys the Vi coating, or the untreated suspension may be agglutinated by a Vi antiserum (prepared from alcohol-treated strains). Possession of the Vi antigen is linked with enhanced virulence.