Human recombinant granulocyte-macrophage colony-stimulating factor (hrGM-CSF) stimulated granulocyte-macrophage (GM) colony formation from human marrow mononuclear cells (MMCs) in a dose-dependent manner in methylcellulose culture. When phagocytes were depleted from MMCs, GM colony formation from the phagocyte-depleted (PD) MMCs by hrGM-CSF markedly decreased. Experiments in which PD-MMCs were cultured with hrGM-CSF and adherent cells showed that 94% (on day 7 and day 14) of the colonies from PD-MMCs were dependent on the presence of adherent cells. In contrast, the ability of granulocyte colony-stimulating factor (G-CSF) to form colonies was not affected by phagocyte depletion. To check for the presence or absence of progenitors that could form GM colonies in direct response to hrGM-CSF, single-cell culture of hematopoietic progenitor cell surface antigen (My-10)- positive PD-MMCs was carried out using a flow cytometer and an Autoclone System. In duplicate experiments, 0.7% and 3.5% (day 7) or 3.6% and 3.9% (day 14) of My-10-positive PD-MMCs formed GM colonies in response to hrGM-CSF and 5.1% and 6.0% (day 7) of My-10-positive PD- MMCs formed GM colonies in response to G-CSF. This was clear evidence for the presence of progenitors directly responding to hrGM-CSF. Also observed was a synergistic effect on GM colony formation in which more My-10-positive PD-MMCs stimulated by hrGM-CSF and G-CSF could form GM colonies than the sum of those stimulated by each separately. This enhancing effect of colony-forming activity of hrGM-CSF by adherent cells and the single cell culture experiment were reproduced in serum- free culture system.