Cultured mononuclear phagocytes produce soluble factors that stimulate endothelial cells to release GM-colony-stimulating activity (GM-CSA). One such factor was recently identified as interleukin 1 (IL 1). Studies were designed to determine which types of granulopoietic factors are released by IL 1-stimulated endothelial cells. Supernatants from endothelial cells cultured for 3 days in medium containing IL 1 alpha and beta were tested in both murine and human CFU-GM colony growth assays. The effect of conditioned media on differentiation of WEHI-3B myelomonocytic leukemic cells was also examined. Control media containing IL 1 alone or unstimulated endothelial cell-conditioned media contained no detectable CSA in any bioassay. Medium conditioned by IL 1-stimulated endothelial cells stimulated the clonal growth of both human and murine CFU-GM and induced macrophage differentiation of WEHI-3B cells. Treatment of these conditioned media with a highly specific neutralizing monoclonal G-CSF antibody completely inhibited their activity in the murine CFU-GM assay, but only partially inhibited GM colony growth by human marrow. Treatment of the active conditioned media with a neutralizing rabbit anti-human GM-CSF antibody partially reduced the activity of the media in the human GM-colony growth assay. G-CSF radioimmunoassay of endothelial cell culture supernatants and Northern blot analysis of endothelial cell cytoplasmic RNA for GM-CSF gene transcripts confirmed that IL 1 induced expression of both G-CSF and GM-CSF genes. Because treatment of media with both antibodies abrogated all activity in the human GM colony growth assay, we conclude that IL 1-stimulated endothelial cells release both G and GM-CSF and that these are the only granulopoietic factors detectable in clonogenic assays released by these cells in vitro.