A polysaccharide derivative comprising a trisaccharide repeating unit consisting of N-acetyl-beta-D-mannosaminouronic acid, 3,N-diacetyl-alpha-L-fucosamine and N-acetyl-beta-D-fucosamine residues linked sequentially (1->4) and (1->3), with the trisaccharide repeats themselves being linked (1->4); corresponds to the serotype 5 (CP5) capsular polysaccharide of Staphylococcus aureus.