Human recombinant GM-CSF (rGM-CSF) was tested on highly purified and fractionated CFU-GM subsets. The fractionation was performed with the DS1-1 monoclonal antibody (MoAb), which distinguishes early and late CFU-GM. On whole bone marrow cells, rGM-CSF had a colony-stimulating activity comparable to that of known sources of CSFs, ie, the supernatant (SN) of TPA 30-1 or 5637 cell lines, used as control. A greatly reduced activity was observed when CFU-GM were depleted of phagocytizing and E rosetting cells (colony growth of 27% as compared with control). On fractionated CFU-GM, the rGM-CSF activity was even more reduced on both early and late progenitors (18% and 6% of colony growth, respectively). However, when rGM-CSF was used together with rG-CSF at suboptimal concentrations, the colony growth reached values analogous to that of control cultures. A synergistic interaction between rGM-CSF and rG-CSF in stimulating either early or late myeloid progenitors was observed. The results suggest that the activity of rGM-CSF on CFU-GM is mainly exerted through cooperation with accessory cells. r-G-CSF is one of the factors that can synergistically cooperate with r-GM-CSF in the myelopoietic stimulation.