In this study we used a long-term culture system to evaluate engraftment potential of human peripheral blood (PB) cells mobilized by chemotherapy (CT) associated or not with granulocyte-macrophage colony-stimulating factor (GM-CSF). In six patients who underwent blood cell transplantation, PB CD34+ cells were cultured after mobilization and were compared to CD34+ cells in steady state from PB and bone marrow (BM). Qualitative differences were shown between PBC samples obtained after CT with and without GM-CSF. Despite similar CFU-GM counts at culture initiation, GM-CSF-mobilized CD34+ cells might contain a lower proportion of primitive stem cells, as suggested by the significant decrease in CFU-GM numbers produced beyond week 5 compared to CT-mobilized CD34+ cells (p = 0.033). Likewise, the percentage of CFU-GM produced beyond week 5 in relation to initial input was significantly lower than steady-state PB (p = 0.039) and than CT-mobilized CD34+ cells (p = 0.033). However, this CFU-GM production with GM-CSF-mobilized PB CD34+ cells was not different from cultures with BMC CD34+ cells. These results suggest that GM-CSF can mobilize CFU-GM in the blood mainly by differentiation at the expense of the primitive stem cell compartment. It appears valuable to define clearly for each mobilizing procedure a particular threshold of CFU-GM which reflects sufficient numbers of primitive stem cells to ensure long-term engraftment.