A polysaccharide derivative comprising a trisaccharide repeating unit consisting of 2-acetamido-4-amino-2,4-dideoxy-alpha-D-fucose and two alpha-D-galacturonic acid residues linked sequentially (1->4) and (1->3) and with the trisaccharide repeats themselves being linked (1->3); corresponds to the serotype 1 capsular polysaccharide (CPS) of Streptococcus pneumoniae.