In the studies reported here we demonstrate that osteopontin is secreted from the basolateral surfaces of osteoclasts where it binds to the avb3-integrin, suggesting that it may be an autocrine factor. Osteopontin stimulation of osteoclasts produced changes in cell shape by causing disruption of peripheral podosome structures and formation of actin filaments at the leading edge of the migrating osteoclasts. The latter was part of the assumption of a motile phenotype prior to cells reforming peripheral ring type podosome containing clear zones. It is well established in our laboratory as well as in others that osteopontin stimulated osteoclast motility and bone resorption. The effect of osteopontin was mimicked by RGD containing peptides and blocked by a avb3 antibody, demonstrating that signals generated by integrin ligation contributed to the actions of osteopontin. In addition, the migratory effects of osteopontin on osteoclasts were also mediated through CD44 receptors since blocking antibodies to CD44 blocked stimulation of motility. Our data strongly suggest that osteopontin is an osteoclast autocrine motility factor binding to avb3 and CD44 during stimulation of osteoclast migration.