In several different strain combinations of rats, an R2 backcross was used to assess the relative fitness of progeny MHC-incompatible with their mothers as compared to that of MHC-compatible progeny. None of the various treatments used to alter maternal immunoregulatory processes, namely splenectomy, active or adoptive immunization, or the induction of transplantation tolerance, was able to perturb consistently the population distribution of RT1 antigens. Though the extreme variability of RT1 heterozygote excess remains poorly understood, studies of parity-associated changes in the population distribution of RT1 heterozygotes: RT1 homozygotes nonetheless do support the hypothesis that the spleen is important to the immunoregulatory control of these phenomena.