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The relationship between nasopharyngeal carriage of Streptococcus pneumoniae types 3, 19, and 23 and the development of homotypic serum antibody was examined in 17 infants followed prospectively from birth. Serially drawn sera from these children and single serum samples from 18 normal adolescents were assayed for opsonizing antibody using reduction of nitroblue tetrazolium to measure phagocytosis of killed whole pneumococci. Selected sera were also tested for quantitative antibody level by a radioimmunoassay, which correlated quite well with the nitroblue tetrazolium assay. All but one of the adolescents had antibody to type 19, and all 18 had antibody to type 23. None of the 12 infants tested for antibody to type 19 pneumococci showed evidence of an antibody response. Two of 10 infants responded to type 23 pneumococci, as did two of three infants tested to type 3 pneumococci.