SummaryGenetically based differences in the antibody responses to the large intestinal nematodeTrichuris muriswere studied in two groups of H–2 congenic strains of mice that differed in their relative resistance to infection with this parasite. The primary antibody response to parasite excretory/secretory (E/S) antigen was predominantly an IgG response with the strains forming two distinct groups, defined by their genetic background. The more susceptible B10 genetic background mice had strikingly higher antibody levels than mice of the BALB genetic background. Superimposed upon these background effects were clearly defined influences attributable to H–2–linked genes, strains which differed genetically only at H–2 loci exhibiting differences in the kinetics of the antibody response. Only B10.G and B10.BR mice showed any great increase in IgM levels post–infection. No IgA specific to E/S antigen was detected in the peripheral circulation of any strain at any time post–infection. Antibody responses to a 40–43 kD antigen revealed clear H–2–linked gene effects, with mice sharing the H–2khaplotype (B10.BR, BALB/K) exhibiting considerably higher total antibody levels than strains expressing other haplotypes; mice of the H–2dhaplotype (BALB/c, B10.D2/n) responded very weakly to this antigen. A Western blot analysis of antigen recognition by antibody revealed similarities between the mouse strains in their total antibody responses toT. murisE/S antigen. However, immunoprecipitation studies showed that in general the more susceptible B10 congenic strains had wider spectra of antigen recognition than the BALB congenics. Strains sharing the same H–2 haplotype had dissimilar antigen recognition profiles, but strains sharing the H–2bhaplotype (B10, BALB/B) recognized a low mol. wt antigen (20–23 kD) not recognized by any other strain, suggesting an exclusively H–2brestriction in the recognition of this antigen. These results support the conclusion that both H–2–linked and background genes play important roles in controlling the humoral im