The cytokine interleukin-1 (IL-1) plays a role in the regulation of normal as well as leukemic hematopoiesis. In acute myeloid leukemia (AML), IL-1 induces autocrine granulocyte/macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor (TNF) production, and these factors may then synergistically induce proliferation in AML blast cells. In this report, we show that IL-1 stimulates DNA synthesis of highly enriched normal bone marrow blast cells (CD34 positive, adherent cell depleted, CD3/CD14/CD15 negative). The stimulative effect of IL-1 can be blocked with neutralizing anti-TNF alpha and anti-GM-CSF antibodies and, most efficiently, by the combination of anti-TNF alpha and anti-GM-CSF, but not with anti-G-CSF antibody, suggesting that IL-1-induced proliferation was initiated through TNF and GM-CSF release. Concentrations of TNF and GM-CSF increased in the culture medium of normal bone marrow blast cells after IL-1 induction. Of the IL-1-induced cells, 12% were positive for GM-CSF mRNA by in situ hybridization, as opposed to 6% of non-induced cells. Thus, in addition to its effect on leukemic blast cells, IL-1 also acts on normal marrow blast cells. We propose a scheme where IL-1 stimulation of normal bone marrow blast cells leads to the induction of TNF alpha and GM-CSF, which in association stimulate DNA synthesis efficiently according to a paracrine or autocrine mechanism within the marrow blast cell compartment.