Overlapping synthetic disaccharide, trisaccharide and tetrasaccharide, derived from pneumococcal polysaccharide type 17F (PS17F), were coupled to keyhole limpet haemocyanin (KLH). The conjugates were tested in mice. The disaccharide-KLH and especially trisaccharide-KLH, in combination with Quil A, induced high titres of high-avidity anti-PS17F IgG. Both conjugates protected mice against challenge with Streptococcus pneumoniae 17F. Tetrasaccharide-KLH, although able to elicit anti-tetrasaccharide antibodies, induced a minimal non-protective anti-PS17F IgG response of low avidity. The tetrasaccharide-KLH conjugate, in contrast to the other conjugates, failed to bind rabbit anti-PS17F IgG.