We investigated the effects of murine resident peritoneal macrophages on the in vitro proliferation of erythropoietin (Ep)-sensitive committed precursors colony-forming unit-erythroid (CFU-E) and burst-forming unit-erythroid (BFU-E) with a two-layer cloning system of methylcellulose and semisolid agar. The addition of increasing numbers of macrophages to the agar underlayer resulted in a progressive increase in the numbers of both CFU-E and BFU-E that proliferated in the presence of Ep. CFU-E, but not BFU-E, proliferated to form colonies in the absence Qf exogenously added Ep, and this proliferation was enhanced in a dose-dependent fashion by the presence of macrophages in the underlayer. The enhancing effects of a given number of macrophages and a given concentration of Ep were greater than the sum of the individual effects of macrophages and Ep alone. This erythropoietic syngerism was more evident with BFU-E because burst formation was not seen in the absence of exogenously added Ep. Macrophage underlayers stimulated three to five times the number of erythroid bursts seen with Ep alone. Cell-free agar underlayers or agar underlayers prepared with nonadherent peritoneal cells or unseparated cells from thymus, lymph node, or spleen failed to augment Ep- dependent erythroid colony formation. No enhancement of CFU-E or BFU-E was demonstrable after depletion ofadherent cells from peritoneal cell suspensions by passage over columns of Sephadex G-10. Analysis by sedimentation velocity of peritoneal cells confirmed that the cells responsible for elaborating the erythroid-enhancing activity were macrophages on the basis of morphologic, histochemical, and functional criteria. Serum- free media conditioned by macrophages in the absence of Ep contained the erythroid-enhancing activities, which indicated that Ep was not required for the elaboration of these diffusible substances. These studies indicate that although macrophages are not obligate for the growth of erythroid precursors, they serve as an important source of diffusible factors that reduce the in vitro requirement for Ep.