Electrophoretically pure human interleukin 1 (IL-1) beta was found to stimulate human endothelial cells in monolayer culture to elaborate colony-stimulating activity (CSA). Supernatant fluids from cultures stimulated with increasing concentrations of IL-1 were found to stimulate colony formation of myeloid (CFU-GM), erythroid (BFU-E), and multipotent (CFU-GEMM) progenitor cells in a dose-dependent fashion. The effect on mixed colony formation, however, was less than on CFU-GM and BFU-E growth. Similar to IL-1, the double-stranded RNA polyriboinosinic-polyribocytidilic acid (poly[rI].poly[rC]) also stimulated release of CSA by endothelial cells in a dose-dependent manner. The kinetics of IL-1-induced CSA release as opposed to poly(rI).poly(rC)-induced release were found to be different, in that poly(rI).poly(rC)-induced CSA production occurred more slowly. An anti-IL-1 beta antiserum was able to completely neutralize the IL-1-induced CSA release, but had no effect on poly(rI).poly(rC)-dependent CSA production, indicating that the latter effect was mediated by other mechanisms than intermediate production of IL-1 beta. Using specific immunologic assays, IL-1- as well as poly(rI).poly(rC)-inducible production of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF, and macrophage CSF was found. The release of CSF from endothelial cells in response to IL-1 may be a mechanism for stimulating production of neutrophils and mononuclear phagocytes, and for attracting and activating these cells at sites of inflammation.