The purpose of this study was to evaluate differences in the safety and immunogenicity of Haemophilus influenzae type b (Hib) capsular polysaccharide vaccine produced by two manufacturers (Connaught and Praxis) in children 18 and 24 months of age. Eighty-five children were evaluated in a prospective, doubleblind, randomized fashion. Postvaccination antibody concentrations (measured by radioimmunoassay) and response rates were not significantly differ-ent between the two manufacturers' vaccines but immunogenicity was significantly less in 18-month-old children (antibody concentration, 0.149 jug/ml) compared with 24-month-old children (0.838 iig/ml) (P = 0.001). No significant differences were noted in the safety of the two vaccines. This study suggests that previously observed differences of immunogenicity data between various type b capsular polysaccharide vaccines are due to differences in antibody assays, not in vaccines. Eighteen-month-old children appear to have a relatively poor immune response to type b capsular polysaccharide. Therefore to optimize the benefits of immunization, we suggest children receive this vaccine at 24 months of age.