Depletion of macrophages from murine marrow by the use of a monoclonal anti-macrophage antibody resulted in a significant increase in the number of erythroid burst forming units (BFU-E). This increase could be neutralized by the addition back to culture of macrophages or macrophage conditioned medium indicating that the suppression was mediated by soluble factors. To further characterize this effect, the addition to culture, either alone or in combination, of interleukin-1 alpha (IL-1 alpha), tumor necrosis factor alpha (TNF alpha), and granulocyte-macrophage colony-stimulating factor (GM-CSF) on the growth of BFU-E and the colony-forming unit granulocyte-macrophage (CFU-GM) was examined in macrophage-containing and macrophage-depleted cultures. The addition of IL-1 alpha to culture stimulated the release of both TNF alpha and GM-CSF and acted synergistically with both cytokines, resulting in a dose-dependent suppression of BFU-E and stimulation of CFU-GM growth. The increase in CFU-GM caused by the addition of IL-1 alpha was mediated by GM-CSF but not by TNF alpha as the increase was prevented by the addition of a monoclonal anti-GM-CSF antibody but not by anti-TNF alpha. When either TNF alpha or GM-CSF was neutralized by monoclonal antibodies the addition of IL-1 alpha resulted in a significant increase in BFU-E growth. The addition of GM-CSF to culture caused a dose-dependent suppression of BFU-E that was mediated by TNF alpha, as colony number was not reduced when GM-CSF and a monoclonal anti-TNF alpha antibody were simultaneously added to culture. TNF alpha- induced suppression of BFU-E only occurred in the presence of macrophages. In macrophage-depleted cultures, a dose-dependent suppression of BFU-E could be induced if subinhibitory concentrations of IL-1 alpha or GM-CSF were simultaneously added with increasing concentrations of TNF alpha. The effects of IL-1 alpha or GM-CSF and TNF alpha were markedly synergistic so that the doses required to induce suppression when added simultaneously was only 10% of that required when either were added to culture alone. Suppression of BFU-E by GM-CSF or the combined addition of GM-CSF and TNF alpha did not require IL-1 alpha because inhibition was not neutralized by the addition of anti-IL-1 alpha antibody.