Bone marrow transplantation constitutes one of the strongest possible challenges to the mechanisms of transplantation immunity. Because the transplanted marrow can restore both hematopoietic and immunologic function, extremely powerful immunosuppressive techniques can be used to facilitate engraftment. However, despite the effectiveness with which the host immune response is ablated, the development of graft-versus-host disease by the newly implanted donor immune system exposes every host tissue to the mechanisms of immunologic rejection. In this setting antigenic disparities expressed on any tissue can influence the outcome of the trans plantation procedure. All attempts to transplant human marrow except between identical twins involve such antigenic disparities and therefore could be described as histoincompatible. This review surveys the results of human marrow transplantation where donor and recipient are not genotypically identical for all detectable anti gens of the major human histocompatibility complex (MHC)-the human leukocyte antigen (HLA) system.