Administration of stem cell factor (SCF) has been proven to enhance cytokine-induced mobilization of CD34+ hematopoietic progenitor cells (HPC) into the peripheral blood (PB). The aim of the present study was to explore in a homogeneous group of 22 uniformly treated breast cancer patients: (1) the kinetics of mobilization into PB of both CD34+ and CD34- cell subsets, including dendritic cells, in sequential samples obtained from day +7 up to day +12 after mobilization; and (2) the composition of the CD34+ and CD34- cell subsets present in the two leukapheresis products obtained for each patient. The following CD34+ and CD34- subsets were analyzed: early CD34+ HPC, erythroid-, myeloid- and B-lymphoid-committed CD34+ precursor cells, mature T, B and NK cells, monocytes, neutrophils, eosinophils, basophils, and dendritic cells (DC) including three subsets of lin-/HLADR+DC (CD16+, CD33high and CD123high). Our results show that the absolute number of PB CD34+ HPC progressively increases from day +7 onwards. As far as the CD34- PB leukocyte subsets are concerned, monocytes (CD14+) displayed the earliest recovery after mobilization predicting neutrophil recovery 1 day in advance. The number of CD34+ HPC collected in a single leukapheresis product was always > or = 1.4 x 10(6) cells/kg body weight. No significant changes were observed between the two leukapheresis sessions either as regards their composition in CD34+ HPC subsets or their CD34- leukocyte populations except for a higher ratio of both CD34+ erythroid/CD34+ myeloid HPC (0.35 +/- 0.13 vs 0.30 +/- 0.13; P = 0.04) and neutrophils/monocytes (1.58 +/- 2.1 vs 0.69 +/- 0.27; P = 0.009) found for the first leukapheresis. Interestingly, the overall number of dendritic cells (DC) was higher in the second leukapheresis (1.06 +/- 0.56 vs 1.9 +/- 0.46; P = 0.02) due to a selective increase of the CD16+ antigen-presenting cells. In summary, our results show that the combination of cyclophosphamide, G-CSF and SCF is highly effective for stem cell mobilization, with differences observed in the mobilization kinetics of the different hematopoietic cell subsets analyzed.