The lack of donor availability is a major limitation to the widespread use of allogeneic hematopoietic stem cell transplantation, and therefore it would be beneficial to identify less immunogenic HLA mismatches. The maternal and fetal antigens that are transmitted through the bidirectional transplacental passage during pregnancy may induce tolerance to noninherited maternal antigens (NIMAs) in offspring and to inherited paternal antigens (IPAs) in the mother. Using mouse models of bone marrow transplantation (BMT), we found that a "child-to-mother" BMT from a NIMA-exposed donor reduced the morbidity and mortality of graft-versus-host disease in an antigen-specific manner; however, a "mother-to-child" BMT from an IPA-exposed donor did not. The NIMA-complementary BMT preserved the graft-versus-leukemia effects and favored the immune reconstitution, thus resulting in a marked improvement of the outcome after BMT. These tolerogenic NIMA effects were completely abolished by the depletion of CD4+ CD25+ cells from the donor inocula, thus suggesting the involvement of CD4+ CD25+ regulatory T cells in the tolerogenic NIMA effects. Our findings may therefore have profound implications on the performance of clinical BMT while also potentially helping to develop new strategies for using a NIMA-mismatched donor in the absence of an HLA-identical donor.