We have studied the effects of GM-CSF on cell growth in Dexter's type normal human long-term bone marrow cultures. Non adherent and adherent cells were weekly harvested and studied all over the time of culture up to 10 weeks: number, cytology, hematopoietic granulocyte-macrophage progenitors content (CFU-GM). Control consisted of cultures without growth factor. GM-CSF (100 ng/ml) induced a significant increase in the number of non adherent and adherent cells, mainly increasing the number of cells of the granulocytic lineage. GM-CSF also induced a transient increase in the number of CFU-GM in the non adherent fraction, but as from week 6 of culture, there were no more CFU-GM. At last GM-CSF inhibited completely or partially the adipocyte growth in the adherent stromal cell layers. In conclusion, at this concentration, GM-CSF might be responsible for the accelerated aging, maybe linked to the hematopoietic stem cells exhaustion and to the almost exclusive presence of monocyte-macrophage cell types in the culture supernatant.