We present evidence that macrophages stimulate red cell production by human bone marrow cells in vitro and that this effect is mediated by soluble substances. Bone marrow cells from donors were cultured in liquid media: after 7-14 days, cells highly enriched for macrophages and supernatants from these cultures were collected and cocultured in plasma clots with fresh human bone marrow cells (CFU-e) for 7 days or peripheral blood mononuclear cells (BFU-e) for 15 days. When added in numbers generally noted in marrow aspirates, bone marrow cells enriched for macrophages enhanced erythroid colony (EC) formation. Thus, at macrophage concentrations of 0% versus 1%, 2%, 4% and 10%, the numbers of EC/6 x 105 BM cells were: 677 ± 46 (mean ± SEM) versus 846 ± 98, 938 ± 85, 1039 ± 93, and 956 ± 68, respectively. Stimulation was noted until unphysiologically high ratios of macrophages (>15%) were added, when EC formation was progressively inhibited. The same erythroid growth curve was obtained whether macrophages and red cell precursors were from the same or different donors. Nonadherent cells from liquid culture did not modulate in vitro erythroid growth, whereas adherent cells at low concentrations enhanced and at higher concentrations inhibited EC formation. Supernatants from these macrophage cultures contained a substance, not erythropoietin, with similar erythropoietic effects.