Monocyte colony-stimulating factor (M-CSF, CSF-1) is a macrophage lineage-specific growth factor. Northern blot analysis using a human M- CSF cDNA probe and a specific bioassay for human M-CSF were used to investigate the cellular sources of M-CSF. Expression of the M-CSF gene was induced in blood mononuclear cells stimulated by phorbol myristate acetate (PMA) or gamma-interferon. When mononuclear cells were fractionated into highly purified populations of T cells and monocytes, M-CSF transcripts were detected predominantly in monocytes. Further, monocytes stimulated with PMA released an M-CSF-like activity. These results demonstrate that M-CSF can be produced by activated cells of the macrophage lineage.