As might be anticipated, the major problem with sub-ablative conditioning is that the graft failure rate is increased, with published figures of 5-30% versus 1-5% predicted in fully ablated patients. The incorporation of lymphodepleting antibodies in the preliminary conditioning regimen may allow these rejection rates to be diminished. Moreover, a highly efficient lymphodepleting MAb or MAb combination might be successfully substituted in part or in whole for cytotoxic and immunosuppressive drugs, further increasing the safety and efficacy of the subablative approach to stem cell transplantation. Our own data using the crude polyclonal mixture of antibodies in ATG as a component of pre-transplant conditioning revealed an improvement in engraftment during matched unrelated donor transplantation.The lymphodepleting monoclonal antibody Campath IH has many of the properties desired for this application, and we propose to incorporate it in our conditioning regimen. Since CAMPATH1H persists after infusion, we would expect it to have additional anti-GvHD effector function, further reducing treatment related mortality (TRM).