Endotoxin induces the appearance of granulocyte colony stimulating factor (G-CSF) and macrophage (M)-CSF in the serum of mice. No GM-CSF was detectable in endotoxin serum. Phytohaemagglutinin stimulated human T-lymphocytes to secrete three major forms of GM-CSF. All of the haemopoietic growth factors (HGFs) are active in vivo, but the cellular responses are different for each HGF. Prolonged administration of G-CSF increases the number of circulating neutrophils and does not lead to adverse side-effects. Chronic exposure to GM-CSF induces an accumulation of activated macrophages which can cause considerable tissue destruction. Although the 'src-like' oncogenes induce HGF production by chicken myeloblasts, factor-dependent murine cells can be converted to a tumorigenic phenotype without autocrine growth factor production.