An increasing number of diseases may be treated successfully by allogeneic bone marrow transplantation (BMT). Initially used for the treatment of immunodeficiency where a cell series or product is replaced, it has now become routine treatment for many forms of leukemia where the transplant provides the rescue after lethal marrow ablation. Recently, diseases such as thalassemia and other inherited metabolic diseases have also been treated by BMT. Formerly the problems of BMT were mainly concerned with graft versus host disease (GVHD) in HLA-matched transplants with HLA-mismatched ones not being possible as GVHD was usually fatal. Since the development of techniques for T cell removal the incidence of GVHD has greatly diminished. T cell removal has also allowed HLA haploidentical mismatched grafts to be performed successfully for immunodeficiency, but there is still a high graft rejection rate in leukemia. This also occurs to a lesser extent with HLA-matched grafts in leukemia. Furthermore, in certain forms of leukemia, particularly chronic granulocytic leukemia, the relapse rate after T cell-depleted BMT is much higher. Trials of better forms of bone marrow conditioning of the recipient are being attempted in order to prevent graft rejection and leukemia relapse. These include total lymphoid irradiation, heavier irradiation and chemotherapeutic regimens, or the use of in vivo monoclonal antibodies such as CAMPATH 1G or anti-LFA-1 (CD11a). In the future, positive selection of stem cells combined with hemopoietic growth factors may allow engraftment without graft versus host disease. This should become the method of choice for autologous transplantation for malignancy. Two monoclonal antibodies directed against the human progenitor cell antigen 1 (HPCA-1) (CD34) have been used for autologous positive stem cell selection in primates and these cells gave full hemopoietic reconstitution in the animals following lethal total body irradiation.