Streptococcus suis causes septicaemia and meningitis in pigs and occasionally in humans. A major galactose-inhibitable adhesin recognizing the blood group P-related disaccharide Gal alpha 1-4 Gal beta 1--present in the GbO3 glycolipid was identified in S. suis. Two variant adhesins, inhibitable by galactose and N-acetylgalactosamine (type PN) or galactose only (type Po) both preferred the disaccharide in terminal position. The hydrogen bonding patterns were determined using deoxy and other derivatives of the receptor disaccharide, and were compared to that of E. coli PapG396 adhesin. The essential hydroxyls were the HO-4', HO-6', HO-2 and HO-3 hydroxyls; type Po adhesin also weakly interacted with HO-6 and HO-3'. The mechanism differed from that of E. coli which binds to a cluster of five hydroxyls, HO-6, HO-2', HO-3', HO-4' and HO-6'. The purified adhesin had a molecular weight of 18 kDa and an isoelectric point of 6.4. The agglutination of latex-bound purified adhesin was inhibited by the same inhibitors as agglutination with whole bacteria. The adhesin was detected by immunoblot analysis in all 23 S. suis strains examined representing different serotypes, was highly immunogenic and showed opsonizing activity. This represents the first example of the comparison of the saccharide receptor hydrogen bondings of two bacteria of different origin and shows that the same saccharide may be recognised by two different mechanisms. As a potential virulence factor present in different serotypes the adhesin represents a potential vaccine against S. suis infections.