A defect in antibody response to immunization with Haemophilus influenzae type b (Hib) capsular polysaccharide vaccine has been reported in children with recurrent infections and normal immunoglobin levels. We identified 15 children, aged 2 to 6 years, with this defect, and we evaluated their response to immunization with an Hib capsular oligosaccharide diphtheria CRM197 protein-conjugate vaccine (HbOC). The children received a series of three vaccines: HbOC at 0 and 8 weeks, and the Hib polysaccharide vaccine at 16 weeks. Levels of antibody to the Hib capsular polysaccharide (polyribosyl ribitol phosphate, PRP) and to diphtheria toxoid were obtained before and 4 weeks after each vaccination. The geometric mean serum anti-PRP concentration was 0.17 microgram/ml before immunization and 29.3 micrograms/ml after the second HbOC immunization (week 12). All 15 children had postvaccination anti-PRP antibody levels greater than 1.0 microgram/ml after receiving the second HbOC (week 12). In addition, booster responses were observed after the second Hib conjugate in 13 of the patients and after the Hib polysaccharide in four of the patients. All patients with low preimmunization diphtheria titers had a response to the diphtheria toxoid. These results suggest that conjugation of Hib polysaccharide with diphtheria CRM197 overcomes the defective antibody response to Hib oligosaccharide in children who are initially observed with recurrent infections and inability to respond to the Hib polysaccharide vaccine.