Smooth- and rough-form lipopolysaccharides from phenol-water extraction of cells from Helicobacter pylori type strain NCTC 11637 were isolated as the water-soluble component of high-M(r) and water-insoluble low-M(r) gel. Structural investigations were performed on the intact water-soluble smooth-form lipopolysaccharide, various oligosaccharides formed as chemical and enzymic degradation products, and three oligosaccharide fractions liberated by acetic acid hydrolysis from the water-insoluble rough-form lipopolysaccharide. A structure is proposed for the complete polysaccharide component of the smooth-form lipopolysaccharide comprising the O antigen chain, an intervening region, and the inner core oligosaccharide on the basis of 1H and 13C NMR experiments, fast atom bombardment/mass spectrometry, and methylation linkage analysis of permethylated oligo- and polysaccharide derivatives. The most striking feature of the O antigen region in the lipopolysaccharide is the presence of extended chains with fucosylated and nonfucosylated N-acetyllactosamine (LacNAc) units that mimic human cell surface glycoconjugates in normal human granulocytes. The chains are terminated by di- or trimeric Lewis(x) (Le(x)) determinants, which are also found in tumor-associated carbohydrate antigens in many adenocarcinomas.