Sixty-two infants and 31 toddlers were vaccinated with the tetravalent pneumococcal conjugate vaccine PncOMPC consisting of the capsular polysaccharide of pneumococcal types 6B, 14, 19F, and 23F conjugated to the outer membrane protein complex of Neisseria meningitidis. Infants were vaccinated at 2, 4, and 6 months (group A) or at 4, 6, and 14 months (group B); toddlers were vaccinated at 24 or at 24 and 26 months of age. The IgG responses to the four pneumococcal polysaccharide types were measured by EIA. In infants, types 14 and 19F induced a significant response after the first dose and types 6B and 23F after the second dose. A clearcut booster response was seen to the booster dose given at 14 months, indicating immunologic priming by the primary series at 2-6 months of age. The responses of the toddlers to one or two doses of the vaccine were very similar to the responses in infants.