A polysaccharide derivative with a repeating unit consisting of alpha-D-galactosyl, 2-O-acetyl-beta-L-rhamnosyl, beta-D-glucosyl and N-acetyl-beta-D-galactosaminyl residues linked sequentially (1->3), (1->4) and (1->3), to the galactosyl and N-acetyl-beta-D-galactosaminyl residues of which are attached respectively a beta-D-galactosyl residue and an N-acetyl-alpha-D-glucosaminyl-(1->2)-alpha-L-rhamnosyl disaccharide unit via (1->2) and (1->4) linkages respectively, with all repeating units being linked (1->6). The repeating unit corresponds to that of the capsular polysaccharide of Streptococcus pneumoniae serotype 7F (ST7F).