Tumor necrosis factor (TNF) is synthesized by macrophages exposed to endotoxin. It produces haemorrhagic necrosis of a variety of tumours in mice and is cytostatic or cytocidal against various transformed cell lines in vitro, but viability of normal human or rodent cells is unaffected. The role of TNF is unlikely to be restricted to the rejection of tumours. Colony-stimulating factors (CSFs) are required for survival, proliferation and differentiation of haematopoietic progenitor cells. The haematopoietic growth factor known as granulocyte-monocyte colony-stimulating factor (GM-CSF) has the ability to stimulate proliferation and differentiation of normal granulocyte-monocyte and eosinophil stem cells and enhance the proliferation of pluripotent, megakaryocyte and erythroid stem cells. In addition, GM-CSF stimulates a variety of functional activities in mature granulocytes and macrophages, for example inhibition of migration, phagocytosis of microbes, oxidative metabolism, and antibody-dependent cytotoxic killing of tumour cells. We show here that TNF markedly stimulates production of GM-CSF messenger RNA and protein in normal human lung fibroblasts and vascular endothelial cells, and in cells of several malignant tissues.