Approximately 16,000 children, from two months to five years of age, were vaccinated with the capsular polysaccharide of either Haemophilus influenzae type b or group C Neisseria meningitidis. Immunizations were carried out in a double-masked, randomized manner; the doses of immunogens used were 10 microgram of H. influenzae type b polysaccharide and 25 micron g of the group CN. meningitidis polysaccharide. Immunogenicity of the two vaccines was measured in single, random specimens of blood taken from vaccinees of all ages at various intervals after immunization. A positive effect on formation of serum antibody was observed in children of all ages vaccinated with N. meningitidis polysaccharide, but increased levels of serum antibody to H. influenzae type b were observed only in recipients of that vaccine who were three years of age or older. No untoward reactions to either vaccine were noted, and both vaccines retained their original molecular size after storage for three years. Too few cases of disease have been studied for a definitive assessment of vaccine efficacy; however, a slightly protective effect against meningitis was observed for the H. influenzae type b vaccine in infants up to one year of age.