The article provides a review of the role of granulocyte colony-stimulating factor (G-CSF) for mobilization and transplantation of peripheral blood progenitor and stem cells. Recombinant gene technology has permitted the production of highly purified material for therapeutic use in humans. Progenitor cells can be assessed using semisolid and liquid culture assays or direct immunofluorescence analysis of cells expressing CD34. This antigen is found on lineage-determined hematopoietic progenitor cells as well as on more primitive stem cells with extensive self-renewal capacity. Administration of G-CSF during steady-state hematopoiesis or following cytotoxic chemotherapy leads to an increase of hematopoietic progenitor cells in the peripheral blood. The level of circulating CD34+ cells post-chemotherapy is greater compared with G-CSF administration during steady state. On the other hand, CD34+ cells harvested post-chemotherapy contain a smaller proportion of more primitive progenitor cells (CD34+/HLA-DR- or CD34+/CD38-) compared with G-CSF treatment alone. Independent of the mobilization modality, the amount of previous cytotoxic chemo- and radiotherapy adversely affects the yield of hematopoietic progenitor cells. While continuous subcutaneous administration of G-CSF between 5 and 16 micrograms/kg bodyweight is preferred, additional dose-finding studies may be helpful to optimize current dose schedules. Adhesion molecules like L-selectin, VLA (very late antigen)-4 and LFA (leukocyte function antigen)-1 are likely to play a role in mobilization, since these antigens are expressed on CD34+ cells from bone marrow in different densities compared with blood-derived CD34+ cells collected following G-CSF-supported cytotoxic chemotherapy. It is also relevant for transplantation that during G-CSF-enhanced recovery post-chemotherapy, peripheral blood is enriched with a greater proportion of CD34+ cells expressing Thy-1 in comparison with CD34+ cells from bone marrow samples obtained on the same day or before the mobilization therapy was started. The early nature of the CD34+/Thy-1+ cells is very likely since this phenotype has been found on stem cells from human fetal liver and bone marrow and on cord blood cells. As a result, G-CSF-mobilized blood stem cells provide rapid and sustained engraftment following high-dose therapy, including myeloablative regimens. Positive selection of CD34+ cells as well as ex vivo expansion using different cytokines are currently being investigated for purging and improvement of short-term recovery post-transplantation. Future developments include the use of blood-derived hematopoietic stem cells for somatic gene therapy. The availability of growth factors has been an important prerequisite for the development of these new avenues for cell therapy.