Dendritic cells (DCs) are potent antigen-presenting cells that are essential for the initiation of T cell-mediated immunity. DCs develop from myeloid progenitor populations under the influence of granulocyte macrophage colony-stimulating factor (GM-CSF) and pass through an intermediate stage of maturation that is characterized by CD14 expression. Interest has focused on generating human-derived DCs for antigen-specific tumor vaccines to be used as adjuvant immunotherapy in minimal disease settings, such as after autologous transplantation. In the present study, mobilized peripheral blood progenitor cells (PBPCs) were obtained from 18 patients with locally advanced or metastatic breast cancer preparing to undergo autologous stem cell transplantation. PBPCs mobilized in 10 patients with GM-CSF for 1 week, followed by the combination of GM-CSF and G-CSF, were compared with those obtained from patients receiving G-CSF alone with respect to the presence of DC progenitors and the capacity to generate functionally active mature DCs. PBPCs mobilized with GM-CSF/G-CSF were markedly enriched for CD14+ DC progenitor cells as compared with those mobilized with G-CSF alone. Consistent with an immature progenitor population, the CD14+ cells express Ki-67 antigen but not nonspecific esterase. CD14+ cells purified by fluorescence-activated cell sorting from PBPCs mobilized with either regimen and cultured for 1 week in GM-CSF and interleukin-4 generated nearly pure populations of cells with characteristic DC phenotype and function. The addition of GM-CSF to the mobilization regimen resulted in greater yields of functionally active DCs for potential use in posttransplant immunotherapy.