This study investigated the actions of fibroblast growth factor (FGF)-18, a novel member of the FGF family, on osteoblasts, chondrocytes, and osteoclasts and compared them with those of FGF-2 and FGF-10. FGF-18 stimulated the proliferation of cultured mouse primary osteoblasts, osteoblastic MC3T3-E1 cells, primary chondrocytes, and prechondrocytic ATDC5 cells, although it inhibited the differentiation and matrix synthesis of these cells. FGF-18 up-regulated the phosphorylation of extracellular signal-regulated kinase in both osteoblasts and chondrocytes and up-regulated the phosphorylation of p38 mitogen-activated protein kinase only in chondrocytes. FGF-18 mitogenic actions were blocked by a specific inhibitor of extracellular signal-regulated kinase in both osteoblasts and chondrocytes and by a specific inhibitor of p38 mitogen-activated protein kinase in chondrocytes. With regard to the action of FGF-18 on bone resorption, FGF-18 not only induced osteoclast formation through receptor activator of nuclear factor-B ligand and cyclooxygenase-2 but also stimulated osteoclast function to form resorbed pits on a dentine slice in the mouse coculture system. All these effects of FGF-18 bore a close resemblance to those of FGF-2, whereas FGF-10 affects none of these cells. FGF-18 may therefore compensate for the action of FGF-2 on bone and cartilage.Fibroblast growth factors (FGFs) 1 are potent mitogens for a wide variety of cells of mesenchymal and neuroectodermal origin (1). FGFs also play a role in the differentiation of a variety of cells and are involved in morphogenesis, angiogenesis, and development. The FGF family now consists of 23 members, FGF-1 to FGF-23, and there are 4 structurally related highaffinity receptors (FGFR1 to FGFR4) belonging to receptor tyrosine kinases that have an intrinsic protein tyrosine kinase activity and elicit tyrosine autophosphorylation of the receptor (1, 2). Recent reports showing that mutations of FGFRs cause several genetic diseases with severe impairment of bone and cartilage formation, such as achondroplasia (3, 4) and thanatophoric dysplasia type II (5), indicate the essential role of FGF signalings on bone and cartilage metabolism.Among FGFs, FGF-2 is well known as a potent regulator of functions of bone and cartilage cells. It is produced by cells of osteoblastic lineage, accumulated in bone matrix, and acts as an autocrine/paracrine factor for bone cells (6 -8). We and others have reported that the exogenous application of FGF-2 has stimulatory effects on bone formation in several in vivo models as a pharmacological action (9 -11). In addition, the Fgf-2-deficient mouse exhibits decreased bone mass and bone formation, although these changes were rather moderate (12). Paradoxically, FGF-2 is also known as a potent stimulator of bone resorption (13-17) and is involved in joint destruction of rheumatoid arthritis patients (18). The stimulatory effect of FGF-2 on osteoclast formation is mediated by the induction of cyclooxygenase-2, a main regulatory enzyme for prostagland...