A polysaccharide derivative comprising a trisaccharide repeating unit consisting of N-acetyl-beta-D-mannosaminouronic acid, N-acetyl-alpha-L-fucosamine and N-acetyl-beta-D-fucosamine residues linked sequentially (1->3), with the trisaccharide repeats themselves also being linked (1->3); corresponds to the de-O-acetylated serotype 8 (CP8) capsular polysaccharide of Staphylococcus aureus.