Angiogenesis and bone remodeling are closely associated, and vascular endothelial cells may have potential roles for osteoclastic bone resorption. We examined whether clonal endothelial cells established from bone, aorta and brain of Balb/c mice influenced osteoclast-like cell formation in vitro. As low as 1% conditioned media of those endothelial cells inhibited osteoclast-like cell formation in bone marrow cultures induced by 1,25-dihydroxyvitamin D3, and did so in spleen cell cultures in the presence of soluble receptor activator of nuclear factor-kappaB ligand (RANKL), M-CSF and prostaglandin E2. The level of osteoprotegerin (OPG), a decoy receptor for RANKL, secreted by endothelial cells was not high enough to inhibit osteoclastogenesis. These observations suggest that endothelial cells derived from various tissues secrete factor(s) that inhibits precursors to differentiate into osteoclasts even in the presence of optimal stimulators for osteoclastogenesis. Hence, endothelial cells in bone may inhibit recruitment of fresh osteoclasts, and those in tissues other than bone may be involved in prohibiting ectopic osteoclastogenesis.