Granulocyte-macrophage colony-stimulating factor synergizes with interleukin-6 in supporting the proliferation of human myeloma cells. The role of granulocyte-macrophage colony-stimulating factor (GM-CSF) in the growth of multiple myeloma (MM) was investigated in 21 patients with MM. In 17 patients with proliferating myeloma cells in vivo, recombinant GM-CSF significantly increased the endogenous-IL-6-mediated spontaneous myeloma cell proliferation occurring in 5-day cultures of tumor cells in vitro (P less than .01). Furthermore, GM-CSF was detected in 5-day culture supernatants of myeloma bone marrow cells. This endogenous GM-CSF was produced by the myeloma bone marrow microenvironment but not by myeloma cells and contributed to the spontaneous myeloma-cell proliferation observed in 5-day cultures. In fact, this proliferation was partially blocked (67%) by anti-GM-CSF monoclonal antibodies. The stimulatory effect of rGM-CSF was mediated through IL-6 because it was abrogated by anti-IL-6 monoclonal antibodies. rGM-CSF did not reproducibly increase the endogenous IL-6 production in short-term cultures of bone marrow cells of MM patients. Using an IL-6-dependent myeloma cell line (XG-1 cell line), rGM-CSF was shown to act directly on myeloma cells stimulating by twofold their IL-6 responsiveness. rGM-CSF did not induce any IL-6 production in XG-1 cells, nor was it able to sustain their growth alone. Although no detectable GM-CSF levels were found in the peripheral or bone marrow blood of MM patients, it is possible that GM-CSF, produced locally by the tumoral environment, enhances the IL-6 responsiveness of myeloma cells in vivo in a way similar to that reported here in vitro. 