Osteopetrotic mice lacking functional M-CSF recover with ageing, suggesting alternate osteoclastogenesis pathways exist. One alternative is GM-CSF, treatment with which improves the osteopetrosis. Our objective was to determine whether GM-CSF could replace M-CSF in human osteoclastogenesis in vitro. Human CFU-GM precursors cultured with RANKL differentiate into osteoclasts without added M-CSF, indicating constitutive production of M-CSF. Addition of M-CSF antibody completely inhibited differential, demonstrating M-CSF-dependance in vitro. Co-treatment with low concentrations (0.01 ng/mL) of GM-CSF for 14 days or higher concentrations (10 ng/mL) for the first 1-2 days enhanced osteoclastogensis at 3 h but suppressed expression at 7-14 days. Neither FLT3-ligand nor VEGF supported osteoclastogensis in the absence of M-CSF. Thus, in vitro human osteo clastogensis is dependant on M-CSF and the stimulatory effects of GM-CSF are mediated by M-CSF. Rescue by GM-CSF M-CSF-deficiency is unlikely to be directly mediated by FLT3-ligand or VEGF. (C) 2004 Elsevier Inc. All rights reserved.