Ninety-nine college students (58 males and 41 females) aged 17 to 23 years were each injected subcutaneously with 50 micrograms of meningococcal polysaccharides A and C. Titrations of antisera obtained at various time intervals during the two subsequent years were made by passive haemagglutination microtitration using human O Rh-negative red blood cells from a single source. The percentage of responders (those who developed a fourfold increase in titre) to polysaccharide A was 97.9% and that to polysaccharide C was 94.8%. Pre-immunization titres of 16 to polysaccharide A and 8 to polysaccharide C were considered to be threshold values above which the response might be impaired. There was a significant (P less than 0.01) difference in the geometric mean HA titre between vaccinees and control groups after vaccination at each time interval studied. The majorities of both the vaccinees and the controls had a higher peak titre to polysaccharide A than to polysaccharide C.