The interaction of acute myeloblastic leukemia (AML) cells with stromal cells was investigated by adding AML-conditioned media to cultures of human endothelial cells. This conditioned media contained factors that induced expression of both the granulocyte macrophage colony-stimulating factor (GM-CSF) and granulocyte CSF (G-CSF) genes and release of colony stimulating activity from endothelial cells. The conditioned media contained interleukin-1 (IL-1) bioactivity and the endothelial cell stimulatory activity was partially neutralized by anti-IL-1 antiserum. Constitutive expression of the IL-1-beta gene was detected in ten of 17 AML cases analyzed. These results suggest that the unregulated secretion of IL-1 by AML cells can induce stromal cells in vitro to overproduce CSFs. This could contribute to the unrestricted growth of AML cells.