Hemophilus influenzae, type b, antibody frequencies determined with bactericidal and radioimmunoassay tests.

Bactericidal and radioimmunoassay (RIA) antibodies to Hemophilus influenzae, type b, were measured in the sera of 85 children aged 18 through 71 months. Bactericidal antibodies were present in only 21% of those over 60 months; RIA levels rose linearly with increasing age (P less than 0.005). Bactericidal antibodies were present in nine of 29 sera with RIA levels greater than or equal to 0.7 mug/ml and in only four of 56 sera with RIA levels less than 0.7 mug/ml (P less than 0.01). The discrepancies between the two tests probably reflect their measurement of different antibodies. Results of the RIA fit the clinical observation that H. influenzae meningitis becomes less frequent with age.