Recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) promotes the growth of a variety of hematopoietic and nonhematopoietic cells, both benign and malignant. There is now evidence that osteoblast-like cells produce GM-CSF and their growth is stimulated by this cytokine in vitro. We have studied the effect of rhGM-CSF on DNA synthesis and cell proliferation in the human osteogenic sarcoma cell lines U-20S, G-292, MG-63, and HOS. RhGM-CSF stimulated a dose-dependent increase in radioactive thymidine incorporation in each of the four cell lines in the presence of serum-free media, and in two cell lines (HOS and U-20S) in the presence of fetal bovine serum (FBS). In addition, rhGM-CSF produced significant increases in cell proliferation in two cell lines (MG-63 and U-20S) in the presence of 2% FBS. These results suggest that GM-CSF may have an important role in the biology of human osteogenic sarcoma cells. The clinical implications of these findings merit further investigation.