Normal human serum-stimulating activity on granulocyte-macrophage colony formation in vitro

The action of human serum on granulocyte-macrophage progenitors (CFC-gm) from normal human bone marrow has been studied. When human serum was added to a culture of nonadherent bone marrow cells, no colony formation was observed in the absence of exogenous colony-stimulating factor (CSF); however, the number of colonies increased with the addition of exogenous CSF. When serum was added to a culture of unseparated bone marrow buffy coat cells, colony formation appeared even in the absence of exogenous CSF. These results show a synergic effect of serum and exogenous CSF and suggest that adherent cells, when stimulated by a serum component, secrete endogenous CSF.