BACKGROUND: Recently, CD34 antigen expressed on hematopoietic stem cells which is not detected on non-Hodgkin lymphoma (NHL), multiple myeloma and most solid tumors, is identified. In autologous bone marrow transplantation (BMT), positive selection of CD34+ cells may be used to provide hematopoietic stem cells capable of engraftment but depleted of tumor cells. And it can be used to depletion of T lymphocytes to prevent the graft versus host disease (GVHD) in allogeneic BMT. So we performed this study to evaluate the efficacy of purification of CD34+ stem cells with CEPRATE SC Stem Cell Concentration System (CellPro Inc.) and to assess the influence of CD34+ stem cells on engraftment. METHODS: Peripheral blood stem cells were mobilized with cyclophosphamide (except one patient with malignant lymphoma) and G-CSF and harvested using CS-3000 (Fenwall). CD34+ stem cells counted by FACScan (Becton-Dickinson). The conditioning regimens were ICE (Ifosphamide/Carboplatin/Etoposide) in breast cancer, high dose melphalan in multiple myeloma, BEAC (BCNU/Etoposide/Ara-C/Cyclophosphamide) in NHL, TBI (total body irradiation) with cyclophosphamide in acute lymphocytic leukemia (ALL) and busulfan with cyclophosphamide in myelodysplastic syndrome (MDS). We used G-CSF (10 microgram/kg) after transplantation in all patients. RESULTS: Eleven patients, six with high risk or metastatic breast cancer, one with refractory multiple myeloma, one with acute lymphocytic leukemia (transformed from lymphoblasticlymphoma), two with relapsed malignant lymphoma, one with myelodysplastic syndrome (HLA one-locus mismatched allogeneic BMT case, for T lymphocyte depletion) were treated. Hematopoietic stem cells were harvested from autologous peripheral blood in all patients except one patient with MDS whose stem cells were harvested from allogeneic bone marrow. Median duration and number of peripheral blood stem cell (PBSC) harvest were 15 days (13~22) and 3 times (2~8), respectively. The mean number of total stem cells and CD34+ stem cells harvested per pheresis were 204.8 (17.4~797.9)x106/kg and 3.0 (0.3~11.9)x106/kg, respectively. The mean efficacy of CD34+ hematopoietic stem cell selection by CEPRATE SC Stem Cell Concentration System was 47.7% (1.4~99.0%). The number of infused CD34+ stem cells per patient ranged from 0.34 to 4.8x106/kg (mean 2.3x106/kg). After transplantation, the median day of achieving granulocyte counts of >0.5x109/L was 10.5 days and platelet counts of >50x109/L was 14 days. CONCLUSION: CD34+ stem cells separated with CEPRATE SC Stem Cell Concentration System provided reliable and timely hematopoietic reconstitution.