Hematopoietic stem cells circulate in the peripheral blood. These cells can be collected by apheresis techniques either in the unperturbed state, after mobilization following the administration of cytokines like G-CSF or GM-CSF, or during the phase of early blood count recovery following chemotherapy-induced myelosuppression. The number of cells collected following mobilization is greater than that obtained after apheresis in the unperturbed state. There are, however, qualitative differences between unperturbed and mobilized cells. Chemotherapy related mobilization can be potentially dangerous in that severe myelosuppression necessary to achieve mobilization can have serious consequences. There are no controlled studies that evaluate the relative merits of each method of collection. Regardless of the techniques employed peripheral blood stem cells can reliably accelerate hematologic recovery after potentially myeloblative therapy and provide an alternative to bone marrow support.