Irradiated (H-2b X H-2k)F1 and (H-2b X H-2d)F1 recipients strongly resist the growth of H-2b parental bone marrow cells and do not resist marrow grafts from non-H-2b parents such as C3H and BALB/c. This phenomenon of hybrid resistance has been shown to be under genetic control of the H-2D-linked loci and was interpreted by Cudkowicz (9) as due to the existence of H-2D-linked recessive hemopoietic histocompatibility genes. To check whether the H-2D-linked loci are solely responsible for the fate of bone marrow allografts, we measured the strength of resistance of irradiated (B6 X C3H)F1 and (B6 X BALB/c)F1 recipients toward bone marrow grafts from a set of H-2 recombinant and F1 hybrid donors carrying either the H-2b, H-2d, and H-2k alleles. We found that growth of all H-2b grafts was resisted, although to different degrees. Resistance was minimal when donors shared with the input strain of a corresponding F1 hybrid the H-2K and H-2I regions, or when both F1 donors and F1 recipients formed identical unique hybrid Ia molecules. In addition, H-2b grafts were resisted by congenic, H-2D-identical, H-2K-and H-2I-incompatible recipients. The fate of grafts from H-2Dd donors seemed to depend on the incompatibility of the combinatorial determinant Ia.22. If both donor and recipient expressed such a determinant (either in the cis or in the transposition), or if neither could form such a determinant, grafts were not resisted. The H-2Dk allele is not the main or only factor that confers on the C3H parental bone marrow cells the ability to grow unresisted in (B6 X C3H)F1 recipients. Grafts from congenic C3H.OH donors, carrying the same H-2Dk alleles and differing in the left part of the H-2 complex, were resisted by the F1 recipients. We conclude that both class I (K and D) and class II (I-A and I-E) major histocompatibility complex genes, rather than hypothetical hemopoietic histocompatibility genes control hemopoietic resistance. To reconcile codominant inheritance of classic H-2 antigens with the apparent recessive inheritance of hybrid resistance, we assume that there exist parental determinants that are not formed in some F1 hybrids due to preferential association of either Ia alpha chains with allogeneic beta chains or of class I antigens with allogeneic or hybrid class II restriction elements.