The present study focused on whether it is possible to expand monocytic cells from CD34+ progenitor cells by using macrophage colony-stimulating factor (M-CSF) in the absence and presence of mast cell growth factor (MGF) and IL-6. It was demonstrated that CD34+ cells differentiate without expansion to functional mature monocytic cells in the presence of M-CSF or combinations of M-CSF plus IL-6 and MGF. A different response pattern was observed for the number of clonogenic cells. The addition of IL-6 or both IL-6 and MGF to M-CSF containing cultures resulted in significant higher numbers of colony-forming unit-macrophage (CFU-M) as tested in clonogenic and 3H-thymidine assays. Furthermore, M-CSF plus both IL-6 and MGF appeared to be the most potent combination to preserve the monocytic precursor in cell suspension culture assays. These results indicate that IL-6 and MGF in conjunction with M-CSF affect CD34+ cells especially at precursor level without distinct effect on the more mature stages. Secondly we studied whether M-CSF is only critical for the monocytic lineage or also affects dendritic cell (DC) development. Indeed, we were able to culture CD83+ DC from CD34+ progenitor cells in the presence of M-CSF in conjunction with TNF-alpha, IL-4, and MGF although their absolute number is almost threefold lower than the number of CD83+ cells yielded from GM-CSF plus TNF-alpha, IL-4, and MGF stimulated CD34+ cells.