The specific cell sources and signals for induction of various colony-stimulating factors (CSF) in peripheral blood mononuclear cells (PBMC), purified T lymphocyte and monocyte (Mo) populations have been investigated. In the absence of exogenous activating stimuli, human PBMC, T cells and Mo failed to produce stable cytoplasmic mRNA for CSF for macrophages (M-CSF or CSF-1), for granulocytes (G-CSF), for granulocytes and macrophages (GM-CSF) and for multilineage CSF [multi-CSF, interleukin (IL) 3] and thus failed to release CSF proteins. However, after stimulation with phorbol myristate acetate and phytohemagglutinin, M-, G-, GM- and multi-CSF mRNA became detectable in PBMC, resulting in the secretion of the respective proteins. Identical culture conditions resulted in synthesis of only G- and M-CSF by purified Mo, whereas purified T lymphocytes produced GM-CSF and multi-CSF only. When Mo or T lymphocytes were exposed to recombinant human interferon-gamma or were stimulated by triggering the epitopes recognized by the monoclonal antibodies anti-Tll2 and Tll3, respectively, again disparate CSF expression patterns were found to be associated with both cell species. Moreover, IL2-receptive T lymphocytes showed the same distinct pattern of CSF secretion when activated by recombinant human IL2.