A polysaccharide derivative with a repeating unit consisting of beta-D-galactosyl, beta-D-galactosyl and beta-D-glucosyl residues linked sequentially (1->3) and (1->4), to the galactosyl residue at the non-reducing end 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, with all repeating units being linked (1->6). Capsular polysaccharide of Streptococcus suis serotype 1.