A unique avidin-biotin immunoadsorption system (CellPro CEPRATE SC) has been developed for the rapid clinical scale purification of CD34+ cells from bone marrow, mobilized peripheral blood (PBSC), or cord blood. This system has been used to treat more than 2500 patients worldwide in a variety of clinical studies. In the autologous transplantation setting, selection for CD34+ cells results in significant reduction (2- > 5 logs) of contaminating tumor cells in the enriched fractions, as documented using sensitive immunocytochemistry staining techniques or quantitative PCR analyses. In the allogeneic bone marrow or PBSC transplant setting, using the CEPRATE SC system as a method for T cell depletion has shown a 3-log reduction in overall T cell content, with more rapid hematopoietic recovery after PBSC reinfusion as compared with bone marrow. CD34+ enrichment is required for efficient ex vivo expansion of progenitor cells as well as for gene therapy applications involving the genetic modification of stem cells.