The affinities of IgG antibodies to Haemophilus influenzae b capsular polysaccharide (polyribosyl ribitol phosphate [PRP]) elicited 1 month after immunization of 47 infants 2-greater than 18 months of age with a PRP-outer membrane protein conjugate (PRP-OMP) were measured by ELISA. Thirty-four sera had affinities distributed normally about a logarithmic mean of 3.2 x 10(5) l/mol, but 13 samples had undetectable affinities (less than 10(4) l/mol). Median affinities of sera from children 2-6 (1.5 x 10(5) l/mol) and 7-11 months of age (1.6 x 10(5) l/mol) were significantly greater than the median affinities of sera from infants 12-18 (1.8 x 10(4) l/mol) or greater than 18 months of age (4.2 x 10(4) l/mol). Sera from children greater than 18 months of age vaccinated with PRP conjugated to diphtheria toxoid had a median affinity of 6.1 x 10(4) l/mol, equivalent to that of the same age group vaccinated with PRP-OMP. Children vaccinated with PRP conjugate vaccines may produce antibodies of very low affinity, a finding that may have significance for protection from invasive disease.