Either bone marrow or peripheral blood may be harvested to provide hematopoietic stem cells (HSC) for autologous transplantation. Both, however, comprise heterogeneous cell populations. The HSC necessary for successful engraftment constitute a very small fraction of the cells harvested. After collection, the harvested cells usually undergo several processing steps to reduce the product volume, remove cells (such as mature blood cells or tumor cells), or to cryopreserve the cells for later reinfusion. Granulocytes and red blood cells, for example, survive cryopreservation poorly using freezing techniques designed for HSC. Therefore, bone marrows being cryopreserved must be depleted of mature blood cells to avoid toxicity from infusion of damaged mature blood cells. Mature blood cells may also impede the variety of tumor cell purging techniques currently being studied. These processings are designed to minimize the loss of HSC while achieving an appropriate HSC product for the individual patient. A number of apheresis devices and cell washers simplify the enrichment of HSC in the harvested cell products. In contrast, tumor cell purging techniques are not standardized between the various transplant centers.