Vascular endothelial growth factor (VEGF) induces osteoclast differentiation as well as neovascularization by binding to the fms-like tyrosine kinase (Flt)-1 and fetal liver kinase (Flk)-1 receptors. The Flt-4 receptor also plays an important role in angiogenesis and lymphangiogenesis. The purpose of this study was to investigate the functions of Flt-4 in the signaling pathway of osteoclast differentiation. We examined the expression of Flt-4 on osteoclast precursor cells (OCPs), and the ability of recombinant human (rh) VEGF-D, one of the ligands of Flt-4, to stimulate the phosphorylation of extracellular-regulated kinase1/2 (ERK1/2) and to activate the nuclear factor-kappa B (NF-kappaB) pathway in OCPs. The number of osteoclasts induced by injection of rhVEGF-D in osteopetrotic (op/op) mice was also evaluated in the absence or presence of neutralizing antibodies to Flt-4. Flt-4 expression was detected on OCPs at both gene and protein levels and stimulation of Flt-4 by rhVEGF-D might induce activation of mitogen-activated protein kinase (MAPK) and NF-kappaB pathways for induction of osteoclast differentiation. Moreover, the number of osteoclasts in op/op mice increased after injection of rhVEGF-D, but was significantly reduced by the injection of Flt-4 neutralizing antibodies. We have therefore shown that Flt-4 expressed on OCPs, might activate MAPK and NF-kappaB pathways and played an important role in osteoclast differentiation.