Osteoclasts are actively motile on bone surfaces and undergo alternating cycles of migration and resorption. Osteoclast interaction with the extracellular matrix plays a key role in the osteoclast resorptive process and a substantial body of evidence suggests that integrin receptors are important in osteoclast function. These integrin receptors bind to the Arg- Gly-Asp (RGD) sequence found in a variety of extracellular matrix proteins and it is well established that the interaction of osteoclast αvβ3 integrin with the RGD motif within bone matrix proteins is important in osteoclast- mediated bone resorption. In this study, we characterized the effects of two synthetic peptidomimetic antagonists of αvβ3, SC-56631 and SC65811, on rabbit osteoclast adhesion to purified matrix proteins and bone, and on bone resorption in vitro. SC-56631 and SC-65811 are potent inhibitors of vitronectin binding to purified αvβ3. Both SC-56631 and SC-65811 inhibited osteoclast adhesion to osteopontinand vitronectin-coated surfaces and time- lapse video microscopy showed that osteoclasts rapidly retract from osteopontin-coated surfaces when exposed to SC-56631 and SC-65811. SC-56631 and SC-65811 blocked osteoclast-mediated bone resorption in a dose-responsive manner. Further analysis showed that SC-65811 and SC-56631 reduced the number of resorption pits produced per osteoclast and the average pit size. SC-65811 was a more potent inhibitor of bone resorption and the combination of reduced pit number and size led to a 90% inhibition of bone resorption. Surprisingly, however, osteoclasts treated with SC-65811, SC-56631 or the disintegrin echistatin, at concentrations that inhibit bone resorption did not inhibit osteociast adhesion to bone. These results suggest that αvβ3 antagonists inhibited bone resorption by decreasing osteoclast bone resorptive activity or efficiency but not by inhibiting osteoclast adhesion to bone per se.