Haemophilus influenzae type b(Hib) is one of the most common cause of invasive bacterial disease in children. In Korea, However, the proportion occupied by Hib as a cause of pediatric pathogen is less compared to those of western countries. Natural immunity to Haemophilus influenzae type b was determined on 308 Korean children. The titration of antibody to capsular polysaccharide (PRP) was performed by the radioantigen binding assay. Forty percent of the total subjects and 31% of children under 5 years had an antibody level considered to be protective( 0.15 g/ml).Seropositivity was 13.6% in 12~17 month age group, and increased progressively to 19% in 5~10 year age group. In the age groups under 36 months of age, the geometric mean titers of anti-PRP antibody were below the protective level. These data indicate that a lot of Korean children under 5 years of age do not have antibody levels considered to be protective against H. influenzae type b, and do not explain the apparent low risk of Korean children to Hib Further studies including comparative antibody response to polysa-charide and protein antigens with other racial groups are needed.