The antibody titres in 250 subjects, aged 5 to 22 years, who were vaccinated with a mannitol-lyophilized antimeningococcal A + C vaccine, stable only when stored at -20 degrees C, were followed for two years. As measured by indirect haemagglutination (IHA) and indirect immunofluorescence (IF) techniques, titres for both A and C Neisseria meningitidis antibodies remained high. Two years after vaccination titres of antibodies against type A showed fourfold increase over the initial titres in from 46% to 100% of groups of subjects and against type C in from 42% to 80%. For 130 subjects vaccinated with a new lactose-lyophilized antimeningcoccal A + C vaccine (presumed stable at 4 degrees C) antibody titres were measured up to 16 months after inoculation with this vaccine stored at -20 degrees C and also after storage for several periods at 4 degrees C. Antibody titres in all these subjects had fallen to their initial titres by 16 months. The importance of evaluating the results on subjects showing low initial titres (less than or equal to 1/8 as measured by IHA) is discussed, as inclusion of high initial titres influences the extent of the response.