The biosynthesis of the Lewis X determinant (Galβ1-4 [Fucα1-3]GlcNAcβ-) in three strains of Helicobacter pylori has been investigated. Strains UA 861, UA 802 and UA 1182 contain α1,3 fucosyltransferase and β1,4 galactosyltrans-ferase activities that synthesize the Lewis X structure by the transfer of monosaccharides from GDP-fucose and UDP-galactose donors, respectively. The enzyme reaction products that formed were characterized by capillary zone electro-phoresis and by 1H-NMR spectroscopy. The biosynthetic pathway is therefore identical to that found in humans. In the three strains, the fucosyltransferase and galactosyltrans-ferase activities differed in various cellular fractions. Km values for their donor and acceptor substrates also differed.