We compared the serological reactivity of lipopolysaccharides (LPS) isolated from Japanese and Western strains of Helicobacter pylori against anti-Lewis antigen monoclonal antibodies and H. pylori-positive Japanese sera. The two LPS from Western strains (26695 and O:2) did not react with any sera from Japanese patients, while all LPS from Japanese strains and the Sydney strain reacted with these sera. We propose that LPS of all Japanese smooth strains share either one of two epitopes, which are termed highly antigenic and weakly antigenic epitopes, present in the O-polysaccharide portion, and these epitopes are independent the Lewis antigens. The present findings indicated that the two Western strains lacked the two epitopes, which are shared by all Japanese strains.