BACKGROUND: Measurement of postimmunization antibody response to pneumococcal capsular polysaccharide (caps-PS) is the standard method to identify deficiency of antipolysaccharide antibody production. However, no standardized criteria have been defined for classification of patients into responders or nonresponders to caps-PS. METHODS: We vaccinated 37 healthy children and 39 healthy adults with Pneumovax((R)) and measured the anti-caps-PS antibody response to 5 serotypes. We also measured antipneumococcal antibody titers in 82 patients with increased susceptibility to airway infection. The assay was performed according to the 3rd-generation assay format. RESULTS: The lower 5th percentile (cutoff) concentrations for the postimmunization antibody titer in healthy individuals were 0.67 mg/L, 0.45 mg/L, 0.46 mg/L, 0.31 mg/L, and 1.04 mg/L for serotypes 3, 4, 9N, 18C, and 19F, respectively. In 96% of healthy individuals, antibody responses higher than the cutoff concentration were seen for at least 3 of the 5 serotypes. Nine of 82 patients (11%) failed to mount an adequate antibody response for at least 4 of the 5 serotypes tested, whereas only 1 control (1.3%) failed to do so. CONCLUSION: The cutoffs for antibody responses to caps-PS identified in this study appear useful for identifying individuals with an inadequate response to vaccine.