Lipopolysaccharide (LPS) of Salmonella typhi has been analyzed by immunoblotting with pooled sera from typhoid patients. Pooled typhoid sera have recognized all the antigenic determinants of S. typhi LPS, giving a strong reaction with the repeating units on the O-side chains as well as with the core region. Cross-reacting antigens have been observed with the LPS of S. typhimurium and S. enteritidis, while no heterologous reactions were seen with the LPS of E. coli strains.