A simple method for preparing an N. meningitidis complement fixing antigen is described. Hyperimmune rabbit N. meningitidis antiserum reacted approximately the same with CF antigens made from several N. meningitidis groups or strains. Recruits known to have become infected with N. meningitidis strain Bo during recruit training show equal serological responses to CF antigens made from different N. meningitidis groups or strains. Further, 4 recruits who were admitted to the hospital with clinical meningitis also seroconverted equally to any 1 of 6 different N. meningitidis CF antigens. Increase in CF titer (4-fold or greater) to N. meningitidis correlated significantly with acquisition of meningococcus in the nasopharynx as determined by bacterial isolations.