A polysaccharide derivative with a repeating unit consisting of beta-D-galactosyl, beta-D-rhamnoosyl and beta-D-glucosyl residues all linked (1->4), to the galactosyl residue of which is attached an N-acetyl-alpha-neuraminyl-(2->6)-N-acetyl-beta-D-galactosaminyl-(1->4)-N-acetyl-beta-D-glucosaminyl trisaccharide unit via a (1->3) linkage, while to the rhamnosyl residue is linked (1->3) an alpha-D-galactosyl residue, with all repeating units being linked (1->4). Capsular polysaccharide of Streptococcus suis serotype 1/2.