A cohort of infants, previously immunized once or twice between three and 12 months of age with vaccines containing polysaccharide of groups A and C Neisseria meningitidis, received boosters at two and five and one-half years of age with bivalent A/C vaccine. Antibody concentrations were measured by a radioactive antigen-binding assay. Concentrations of antibody to groups A and C N. meningitidis were 5.59 and 2.86 microgram/ml, respectively, by four years of age. After booster immunization at five and one-half years of age, concentrations of antibody to groups A and C N. meningitidis increased to 15.67 and 7.59 microgram/ml, respectively. Protective levels of antibody to group A meningococci may be achievable throughout early childhood by routine immunization with the A vaccine. Although the group C vaccine is effective in control of epidemics, the rapid decline in the concentration of antibody to group C meningococci following immunization of young children suggests that protection may not be long-lasting.