Bone marrow stem cell transplant studies carried out by the NHLBI BMT Unit have focused on approaches to optimize the stem cell and lymphocyte dose in order to improve transplant survival and increase the graft-vs.-leukemia effect. A CD34 stem cell dose of greater than 3 x 10(6)/kg was found to increase survival and reduce relapse, while a CD3+ lymphocyte dose of less than 1 x 10(5)/kg was associated with a very low incidence of GVHD. Although processing of 2 peripheral blood progenitor cell (PBPC) collections with the CellPro immunoabsorption method (combined CD34-positive selection and CD2-negative selection) provided an improvement over previous methods, the system did not always achieve these optimal cell doses. A recent preclinical evaluation by the Department of Transfusion Medicine of a new immunomagnetic cell selection system available from Nexell, Inc. has demonstrated improved recovery of CD34+ cells and increased depletion of T lymphocytes, compared to the CellPro method. Incorporation of the Nexell system (Isolex 300i) into this clinical protocol will allow us to more consistently achieve CD34+ cell doses above the threshold of 3 x 10(6)/kg and CD3+ lymphocyte dosing in the region of 0.5 x 10(5)/kg. This will make it possible to test (1) the potential benefit of optimized transplant cell doses, (2) elimination of post transplant immunosuppression to enhance immune recovery.