Forty-two Gambian children randomised to receive two doses of meningococcal A/C polysaccharide vaccine (MPS) in infancy and either MPS (n = 15), meningococcal A/C conjugate (n = 13) or inactivated polio vaccine (IPV n = 14) at 2 years, were revaccinated with MPS at 5 years of age along with 39 matched control children. Meningococcal A and C polysaccharide antibodies were analysed by ELISA and bactericidal assay (SBA) in sera taken before and 10 days after revaccination. The geometric mean group SBA titre in the MPS group following revaccination was about half that of the unvaccinated controls (0.51 95%CI: 0.28, 0.90) for group A and less than half that of the controls for group C (0.41, 95%CI: 0.16, 1.03 P = 0.06). The group C SBA response in the conjugate group was 14-fold higher than in the MPS group (P < 0.001). Multiple doses of meningococcal polysaccharide in childhood may therefore attenuate the SBA response to both group A and group C polysaccharides. In contrast, vaccination with meningococcal A/C conjugate after MPS in infancy gives immunological memory to N. meningitidis group C.