Granulocyte-macrophage colony-stimulating activity (GM-CSA) can be produced by a variety of normal cell types including mononuclear phagocytes, activated T lymphocytes, endothelial cells, and fibroblasts. Recent evidence shows that a major role of the monocyte-macrophage is the recruitment of environmental cells, i.e., fibroblasts, to produce GM-CSA. In this study we have identified interleukin 1 (IL-1) as a monokine that stimulates fibroblasts to produce and release GM-CSA and prostaglandin E2 (PGE2). Both purified human monocyte-derived IL-1 and human recombinant IL-1 (10(-10) M) can be substituted for monocyte-conditioned medium in stimulating fibroblast GM-CSA and PGE2 production. Both forms of IL-1 stimulate fibroblasts to produce GM-CSA and PGE2 in a dose-dependent fashion. The fibroblast-stimulating activity found in monocyte-conditioned medium was completely blocked by anti-IL-1. We conclude that monocytes produce IL-1, and that monocyte-derived IL-1 induces fibroblasts to produce GM-CSA and PGE2.