Macrophage-derived granulomonopoietic enhancing activity (GM-EA) is a novel mediator that amplifies colony formation of myeloid progenitor cells (CFU-GM) in conjunction with colony-stimulating factors (CSFs), and is distinct from other hematopoietic synergizing factors such as interleukin (IL)-1, IL-4, and IL-6. In the present study, we try to ascertain whether or not there is a GM-EA-specific responsive myeloid progenitor cell population. Human bone marrow cells deleted of adherent cells and T lymphocytes were separated by velocity sedimentation into three subpopulations with respective sedimentation rates (millimeters per hour) of 7.4 +/- 0.4, 6.0 +/- 0.6, and 4.7 +/- 0.3. These subpopulations corresponded to the day 7 CFU-GM, day 14 CFU-GM, and the earlier myeloid progenitor cells, pre-CFU-GM, respectively. Pre-CFU-GM failed to respond to the colony-inducing effect of GM-CSF but could be stimulated by GM-EA alone to generate small clusters (5 to 25 cells) in soft agar after 14 days of incubation. Correspondingly, suspension preculture of the fractionated bone marrow cells also showed that only the progenitor cells with low sedimentation rate (4.7 mm/h) could be activated by GM-EA to generate CFU-GM. Taken together, our results suggest that the specific target cell of GM-EA is the pre-CFU-GM, and that GM-EA acts on these cells as a growth/maturation factor, but on the day 7 and day 14 CFU-GM as a synergistic growth factor.