Abstract Interleukin-1 (IL-1) was found to act synergistically with granulocyte- macrophage colony-stimulating factor (GM-CSF) on granulocytic colony growth of normal human bone marrow cells, depleted of mononuclear phagocytes and T lymphocytes. Using CD34/HLA-DR-enriched bone marrow cells we demonstrated that this activity of IL-1 was not a direct action on hematopoietic progenitor cells, but an effect of an intermediate factor produced by residual accessory cells in response to IL-1. Neutralization experiments using an anti-IL-6 antiserum showed that IL-1-induced IL-6 did not contribute to the observed synergy. Furthermore, IL-6 by itself had neither a direct stimulatory effect on CFU-GM colony growth, nor did it act synergistically with GM-CSF on granulocytic or monocytic colony formation. Neutralization experiments with an anti-G-CSF monoclonal antibody showed that IL-1-induced G-CSF production was responsible for the synergy with GM-CSF. Using combinations of G-CSF and GM-CSF this synergistic activity could be detected at concentrations of G-CSF as low as 0.1 ng/mL (10 U/mL). Our results indicate that IL-1, but not IL-6, stimulates the GM-CSF- dependent proliferation of relatively mature myeloid progenitor cells in the presence of small numbers of accessory cells.