Two recombinant human granulocyte colony-stimulating factors (rHu G-CSF) are clinically available, a glycosylated (lenograstim) and a nonglycosylated from (filgrastim). Since there is accumulating evidence that glycosylation plays a role in the in vitro activity of the G-CSF molecule, we compared the biological potency of lenograstim and filgrastim on human hematopoietic progenitor cells by colony assay in semisolid medium and ex vivo expansion experiments. Leukapheretic products without further processing and CD34-positive purified cells were used as source of human progenitors. Lenograstim demonstrated greater capacity, to stimulate the colony growth of both, purified and CD34+ peripheral blood cells. This effect, which is evident especially at low doses of growth factor, seems not to be mediated by accessory cells. Whether these observations may have clinical relevance is still to be clearly assessed and further investigations are needed.