We investigated the role of normal human marrow cells with Fc receptors for IgG (Fc gamma+) on autologous granulocyte-macrophage colony (GM- CFC) formation. It was found that Fc gamma+ normal human marrow cells, both with (E+) or without receptors for sheep erythrocytes suppressed GM-CFC at as low a concentration as 0.25 X 10(5) cells/ml of culture. A similar effect was observed with E- Fc gamma+ but not E+ Fc gamma+ peripheral blood cells. Suppression by Fc gamma+ cells did not require mitogen activation and was not inactivated by irradiation (2000 R). This report presents a new in vitro regulatory mechanism for GM-CFC growth in normal donors.