The antibody responses of children immunized with Haemophilus influenzae type b polysaccharide vaccine were examined in relation to the absence or presence of the Km(1) or G2m(23) immunoglobulin allotype. Ninety-seven children, 12-83 months of age, were immunized. Sera were obtained before immunization and two months later. Total serum antibody to the type b capsule was detected by a radioactive antigen-binding assay. IgG and IgM antibody responses were measured by enzyme-linked immunosorbent assay. Antibody responses to the type b capsule were more than threefold higher in blacks with the Km(1) immunoglobulin allotype compared with those in blacks lacking this allotype (P less than .02). The isotype affected was IgG (P less than .01) and not IgM. Serum concentrations of IgG2, but not of IgG1, also were higher in blacks with Km(1) (P less than .003). In whites there were no significant differences in the total or IgG-specific antibody responses to the type b capsule in relation to the Km(1) or G2m(23) allotype.