COLONY STIMULATING FACTORS (CSFs) stimulate isolated haemopoietic progenitor cells to differentiate and form granulocyte and/or macrophage colonies in vitro1,2. There seem to be at least two distinct subclasses of CSF, those stimulating macrophage colony formation and those stimulating granulocytic colony formation. Purified CSF from mouse L cell-conditioned medium is a 70,000 molecular weight glycoprotein, comprising two disulphide-bonded 35,000 MW subunits3. At concentrations as low as 10-12 M it stimulates macrophage colony formation by murine bone marrow cells. It has similar physicochemical properties to, and shared antigenic determinants with, the CSF from human urine3-6. This type of CSF has been shown6 to share identity with the macrophage growth factor of Virolainen and Defendi7 that stimulates proliferation of peritoneal exudate macrophages. The purified CSF was also active6 in stimulating macrophage colony formation by peritoneal exudate cells in the agar culture system of Lin and Stewart8. We now report that this same kind of CSF is the molecule responsible for the proliferation of mononuclear phagocytes from several haemopoietic and nonhaemopoietic tissues.