In summary, studies of beta 3-deficient mice demonstrate that they provide an excellent model of the human bleeding disorder, Glanzmann thrombasthenia (GT), and replicate essentially all features of the human disease. Furthermore, these mice reveal additional defects consequent upon the absence of integrins alpha v beta 3 and alpha IIB beta 3 that had not become evident from studies of the human patients. These include defects in placental development and in bone resorption. In contrast, despite published evidence suggesting a role for alpha v beta 3 in angiogenesis, beta 3-null mice have not yet shown any defects in this process. The availability of these mice will allow extensive further studies of GT pathogenesis and therapy and of the roles of beta 3 integrins in different forms of angiogenesis, including the pathologically important angiogenic processes in the retina and in response to tumors. Furthermore, they open the way to studies of a variety of other processes in which roles for beta 3 integrins have been suggested.