The ontogeny of human antibody responses to pneumococcal polysaccharide antigens was studied by determining whether the age at immunization affects the level and/or immunoglobulin isotype of antibody produced. Twenty-nine healthy infants between 2 and 18 months of age and 13 normal adults were studied. Responses were found to vary markedly with the age at the time of immunization and with the pneumococcal serotype tested. Three general patterns of isotype-specific antibody response were observed in the infants: a high response in IgG antibody occurred as early as 2 months of age following immunization with type 3 pneumococcal polysaccharide; little or no response was noted in either IgG or IgM antibody with types 6, 18, and 19; and intermediate responses with IgM antibody increases greater than IgG increases were found for type 23. These data suggest that different factors control the immunologic response of infants to various pneumococcal polysaccharide serotypes following immunization at various ages.