Avidity of antibodies to Streptococcus pneumoniae type 6B, 14, 19F and 23F polysaccharides (PS) evoked by four different pneumococcal conjugate vaccines was compared. Infants were primed with pneumococcal PS conjugated to the variant diphtheria toxin CRM197 (PncCRM), diphtheria toxoid (PncD), tetanus toxoid (PncT) or meningococcal protein complex (PncOMPC) and boosted with the homologous conjugate or PS vaccine. No booster was given to children in the PncOMPC group. Relative antibody avidity was measured by thiocyanate EIA. No vaccine specific differences were found in avidity of anti-14 or -19F antibodies. By contrast, antibody avidity to 6B and 23F differed significantly between the vaccine groups, PncCRM and PncT inducing antibodies of highest avidity.