Specific antibody deficiency lias been recognized as an immunodeficiency. In order to investigate an antipolysaccliaride antibody defect as a cause of rcccttrent infections, 30 children were studied. Patients who had been identified to have a major immunodeficiency or structural abnormality or a disease known to cause infection were excluded."nTliirty patints, aged I to 13.8 years (mean age, 5.8; male; female, 13:17) were chosen; all liad normal IgG and IgGl. Tiic level of IgA, IgG2 and lgG3 were of lower than normal in some cases. All were immunized with Hih conjugate (PRI'-T) vaccine, and 26 with pneumococcal vaccine. Antibody responses were measured 4-6 weeks later. Twelve showed a poor response to immunization: 8 to pneumovax 3 to Hib and 1 to both. No correlation was observed between IgG 2 level anil specific antibody responses to polysacciiaridc antigens. Tlie infections were more severe and more frequent in children who responded poorly to polysacciiaridc antigens. Chikircn who had infections in more than one site were most likely to have deficient antibody responses."nThese results show that assessment of specific antibody responses to challenge immunization is an essential part of the investigation of children suffering from recurrent pyogenic infections. The study confirms that measurement of immunoglobulin isotypes and IgG subclasses atone docs not exclude significant hormonal immune deficiency.