Whether ?1 integrin ectodomains visit conformational states similarly to ?2 and ?3 integrins has not been characterized. Furthermore, despite a wealth of activating and inhibitory antibodies to ?1 integrins, the conformational states that these antibodies stabilize, and the relation of these conformations to function, remain incompletely characterized. Using negative-stain electron microscopy, we show that the integrin ?5?1 ectodomain adopts extended-closed and extended-open conformations as well as a bent conformation. Antibodies SNAKA51, 8E3, N29, and 9EG7 bind to different domains in the ?5 or ?1 legs, activate, and stabilize extended ectodomain conformations. Antibodies 12G10 and HUTS-4 bind to the ?1 ?I domain and hybrid domains, respectively, activate, and stabilize the open headpiece conformation. Antibody TS2/16 binds a similar epitope as 12G10, activates, and appears to stabilize an open ?I domain conformation without requiring extension or hybrid domain swing-out. mAb13 and SG/19 bind to the ?I domain and ?I-hybrid domain interface, respectively, inhibit, and stabilize the closed conformation of the headpiece. The effects of the antibodies on cell adhesion to fibronectin substrates suggest that the extended-open conformation of ?5?1 is adhesive and that the extended-closed and bent-closed conformations are nonadhesive. The functional effects and binding sites of antibodies and fibronectin were consistent with their ability in binding to ?5?1 on cell surfaces to cross-enhance or inhibit one another by competitive or noncompetitive (allosteric) mechanisms.