A polysaccharide derivative comprising a tetrasaccharide repeating unit consisting of a trisaccharide backbone of beta-D-galactosyl, beta-D-glucosyl and N-acetyl-beta-D-glucosaminyl residues linked sequentially (1->4) and (1->6), with a further beta-D-galactosyl residue linked (1->4) to the N-acetyl-beta-D-glucosaminyl residue, and with the tetrasaccharide repeats themselves being linked (1->3); corresponds to the serotype 14 capsular polysaccharide (CPS) of Streptococcus pneumoniae. The structure diagram shows a portion of the polysaccharide chain consisting of three such tetrasaccharide repeating units showing clearly the (1->3) linkages existing between the repeating units.