This study was undertaken to determine whether previously noted differences in the immune response of inbred strains to mice to cholera toxin (CT) might be under immune response gene control. A series of inbred, congenic, and intra-H-21 region recombinant mouse strains were tested for responsiveness to CT after i.p. immunization with 0.1 μg CT in alum. Samples of plasma were collected at intervals before and after priming and boosting. IgG and IgA anti-CT were measured by ELISA. In three different sets of congenic strains, the level of IgG anti-CT clearly depended on the H-2 haplotype of the strain rather than on any background or Igh genes. Strains with the H-2(b) and H-2(q) haplotypes were high responders, and strains with the H-2(k), H-2(s) and H-2(d) haplotypes were low responders. Within the H-2 complex, the IgG anti-CT response was mapped to the I-A subregion with the use of congenic intra-H-2I region recombinant strains. In contrast to these results with IgG anti-CT, plasma IgA anti-CT levels were uniformly low and indeterminate. We conclude that the murine IgG anti-CT response is controlled by a locus within the I-A subregion of H-2 - a remarkable finding, considering the known abilities of this toxin to bind to and to directly stimulate lymphocytes.