CM-S is an autonomous cell line of human hemopoietic precursor cells inducible to monocyte-macrophage differentiation in response to appropriate inducing agents. CM-S cells produce factors that stimulate their own growth and proliferation, and are also capable of stimulating clonal proliferation of human, but not mouse, monocytic and granulocytic bone marrow progenitor cells in viscous medium. Preliminary purification steps have demonstrated at least two species, one of which (MW 30,000-50,000) retains both these activities, while the other (MW less than or equal to 10,000) apparently retains only the autostimulatory activity. CM-S cells could thus be a useful source for the purification of human colony stimulating factors (CSFs). CM-S cells also respond to factors present in human placenta conditioned medium, known to contain human CSF. This suggests that CM-S cells could provide a homogeneous target cell population for testing CSFs from other human sources.