Human antibody to the Haemophilus influenzae capsular polysaccharide (Hib CP) is restricted in diversity in the individual and the population with a limited number of variable region genes encoding antibody. Antibody to the Hib CP shows restricted isoelectric focusing gel patterns and light chain usage with frequent restriction to use of only kappa light chains. Shared cross-reactive idiotypes are expressed on antibody. The heavy chain of antibody to the Hib CP is predominantly encoded by two members of the VH3family—LSG 6.1/M85-like and VH26/30P1-like. In VHthe CDR1, based on complete identity in LSG 6.1/M85-like antibodies, CDR2, based on the suggestion of mutation in this region, and CDR3, based on conserved CDR3 usage in unrelated individuals, may be important for antigen binding. Six or more different VLgene families encode antibody. The predominant antibody of the majority of individuals uses the A2-VκII gene in germline or near germline configuraion, which encodes an idiotype designated Hibld-1. Antibody can also be encoded by VκI, non-A2 VκII, VκIII, VκIV, VλII, and VλVII genes. Although different VLgenes can be used, unrelated individuals appear to use the same VκIII (A27), VλII (Vλ2.1 and VλVII (4A) genes. The VLdiversity accounts for differences in fine binding specificity, with A2-VλII genes not encodingE. coliK100 CP cross-reactive antibodies and VλVII genes and some of the non-A2 Vκgenes encoding cross-reactive antibodies. The arginine in CDR3 of both antibody kappa and lambda light chains and the asparagine in CDR2 of VLsequences and in CDR1 of LSG6.1-M85 VHsequences of antibody appear to be important residues for antigen binding. A relatively limited degree of somatic mutation has occurred in the non-A2 VLgenes, VλVII, and the VHgenes. Further studies comparing the polymorphism of germline V genes to antibody-encoding V genes are needed to clarify this issue. Research comparing this repertoire to repertoires directed to other bacterial CP and to self antigens and defining structure-antigen binding relationships is in progress.