Despite the availability of anti-microbial agents effective against Neisseria meningitidis, meningococcal disease continues to be a major global health problem, particularly in the very young. Serogroup A meningococci cause large epidemics in sub-Saharan Africa, whilst serogroups B and C organisms are responsible for sporadic cases and localised outbreaks of disease world-wide. For measuring functional activity, the serum bactericidal antibody (SBA) assay is the most important method. It is mediated by antibody and complement resulting in lysis of the bacterial cells. To date the SBA has proved to be the best surrogate of protection for all serogroups. For serogroup C, an SBA titre of either >/ or =4 or > or =8 has being utilised for putatively indicating protection when using either human or baby rabbit complement, respectively. For serogroup B, the proportions of vaccines with > or =4-fold rises in SBA pre- to post-vaccination or SBA titres > or =4 have been correlated with clinical efficacy in trials of outer membrane vesicle (OMV) vaccines in Cuba, Brazil and Norway. SBA activity as a correlate of protection for evaluating the immune response to meningococcal vaccines is described in this review.