The present study demonstrates that loss of cell adhesion potently promotes apoptosis in osteoclasts, a process termed "anoikis." When purified mature rabbit osteoclasts were cultured on plastic for 18 h, about 25% of them were spontaneously committed to apoptosis. The death rate increased more than twofold, after osteoclasts were subjected to suspension culture in inverted Terasaki plates. The osteoclast anoikis was significantly prevented by bongkrekic acid, an inhibitor of mitochondrial permeability transition (PT), and z-VAD-FMK, a caspase inhibitor, suggesting involvement of mitochondrial PT and caspase activation in the death process. Colony-stimulating factor-1 (CSF-1), receptor activator of NF-KB ligand (RANKL), and calcitonin protected adherent osteoclasts, but not floating osteoclasts from anoikis. These data show that adhesion is a primary requirement for osteoclast survival.