Two hundred healthy adults were immunized with 1 microgram of the bacterial antigen monomeric flagellin from Salmonella adelaide, and grouped as responders and non-responders on the basis of a rise in titre of antibody 2 weeks after immunization. Immunoglobulin allotypes G1m(a), G1m(z) and G3m(g) were more frequent among responders who made immunoglobulin (Ig)G antibody (P less than 0.02), and HLA-B12 was more frequent among responders who made IgM antibody (P less than 0.05). The mean log titre of IgG antibody was higher in females (P less than 0.001), in subjects with T1m(a), G1m(z) and G3m(g) allotypes (P less than 0.05), and in Gm heterozygotes (P less than 0.01). The mean log titre of the IgG antibody response in subjects with particular Gm phenotypes was also dependent on the HLA-B locus phenotypes HLA-B7, B8 and B12 (P less than 0.005); for example, among those with the phenotype Gm(a-x-b) subjects with HLA-B7 were low responders and those with HLA-B8 were high responders. These findings are consistent with the hypothesis that there are immune response genes within the major histocompatibility complex (MHC) which interact with Gm-linked genes in determining levels of serum antibodies of different isotypes and specificities.