Widespread use of meningococcal A and C polysaccharide (MACP) vaccines has raised concerns about induction of hyporesponsiveness to these polysaccharides. Immunological hyporesponsiveness to C polysaccharide has been clearly documented in infants, children and adults but only limited data from Gambian children are available for A polysaccharide. We investigated whether a second dose of MACP, given 6 months after an initial dose affected the immunological response as measured by the serum bactericidal assay (SBA) and enzyme-linked immunosorbent assay (ELISA), to serogroup A meningococci in young adults (university students, n = 36). Serogroup A SEA responses 1 month following the second dose of MACP (geometric mean titre (GMT) 103.6, 95% CI 45.6-235.1) were approximately one third of the levels observed 1 month post first dose (GMT 281.9, 95% CI 134.9-581.4). The serogroup A-specific IgG levels post second dose (GMC 21.2, 95% CI 15.3-29.4) were also significantly lower at an average of three-quarters the level post first dose (GMC 28.7, 95% CI 20.8-39.7). This confirms that revaccination with MACP vaccine, 6 months following the initial dose, results in a reduced immunological response to A polysaccharide in adults. Repeated vaccination with MACP vaccine may be ineffective and development and use of meningococcal serogroup A conjugate vaccines should be encouraged. (C) 2000 Elsevier Science Ltd. All rights reserved.