Normal infants, selected at six months of age for participation in one of two separate studies of polyvalent pneumococcal vaccines, octavalent vaccine for Eli Lilly Laboratories (Indianapolis, Ind.) and 14-valent vaccine from Merck Sharp & Dohme (West Point, Pa.), were assigned randomly to groups to receive vaccine at six and/or 12 months of age or to control (unvaccinated) groups. Serum collected at ages six, seven, 12, 13, and 24 months provided pre- and postvaccination geometric mean titers (GMTs) as well as information about persistence of antibody titers. Clinical reactions were monitored by a home visit at 24 hr after each injection. The octavalent vaccine, when given at six months of age, stimulated significant antibody against Streptococcus pneumoniae type 3 and against types 3, 7, 18, and 23 when given at 12 months of age; GMTs were significantly higher than those of controls of the same ages (P ⩽0.05). The 14-valent product, given at 12 months of age, was immunogenic against types 6, 7, 8, and 14 when GMTs were compared with those of controls (P ⩽0.05). Vaccination at six months of age was followed by depressed GMTs on revaccination. Natural acquisition of antibody was indicated by rising GMTs in the unvaccinated controls, who had an increase of ⩾1.8-fold for all types during the interval between six and 13 months of age. By the age of 24 months, GMTs of all groups were similar. Clinical reactions were mild and brief (⩽36 hr). Results of these studies indicate that modification of the vaccines is needed for improvement of the immunogenicity in infants.