Colony stimulating factors and especially rHuG-GSF, the first available neutrophil growth factor, have led to considerable interest in the field of stem cell transplantation because of their ability to induce stem cell peripheralization either alone or in association with high-dose chemotherapy. Few data exist, however, on the impact of rHuG-CSF on large scale bone marrow collection and autologous bone marrow transplantation (ABMT). This phase I, non-randomized, dose escalation study of rHu-G-CSF (lenograstim) administered to 30 patients at doses ranging from 1 to 40 micrograms/kg/day for 5 days before bone marrow harvesting showed that priming with rHu-G-CSF in vivo increased the number of bone marrow cells and D14 myeloid restricted progenitors (CFU-GM) and led to a better neutrophil recovery after ABMT compared with a contemporary unprimed control population. Otherwise, this study established that 5 days of rHuG-CSF therapy, as a sole stimulus, induced a tenfold increase in the circulating CFU-GM amongst which immature progenitors, estimated by the Delta assay (secondary CFU-GM grown after 7 days of liquid culture/primary CFU-GM), are detected. These conclusions were valid for doses as low as 2 micrograms/kg/day which induced only mild neutrophilia up to the highest dose (40 micrograms/kg/day) and suggest that a short course of rHuG-CSF is beneficial in increasing the stem cell collection.