IIaemophilus influenzae type b(Hib) was a major cause of bacterial meningitis in young children less than two years old until recently when the effective vaccines became widely used. The V region repertoire of human antibodies to Hib polysaccharide (PS) has been well characterized: Majority of tim adult antibodies against Hib use a VL derived from Vkappa gene A2 and have arginine at the N region. Since the expression of anti-Hib PS antibodies appears to be variable in the different age and ethnic groups, we examined the VL region of antibodies to Hib-PS in Korean population. We immunized Korean adults (n=8) and children (n=39, 6-17 month old) with Hib tetanus conjugate vaccines, isolated RNAs from peripheral lymphocytes, and amplified antibody VL region by RT-PCR. The PCR products were cloned and the cloned fragments were sequenced. Fortyseven out of 54 independent clones from children used Jk2 or Jk3 whereas Jk usage was not restricted in adults. Adults have germline A2-Jk sequences. However, child clones showed consistant variation from the germline A2-Jk sequences. Eightyeight percent (22/25 independent clones) of child clones with A2-Jk2 genes have CGC (arginine) instead of AGC (serine-76) at the base and 77 percent (17/22) of child clones using A2-Jk3 genes have isoleucine-109 at the junction of Jk-Ck instead of threonine found in a germline sequence. These results suggest that the mechanism of antibody production in young children is different from that of adults.