A polysaccharide derivative comprising a tetrasaccharide repeating unit consisting of beta-D-glucose, beta-D-galactose, beta-D-glucuronic acid and beta-D-glucose residues all linked sequentially (1->4) and with the tetrasaccharide repeats themselves being linked (1->4); corresponds to the serotype 8 capsular polysaccharide (CPS) of Streptococcus pneumoniae.