Two disintegrins with a high degree of amino acid sequence similarity, echistatin and eristostatin, showed a low level of interaction with Chinese hamster ovary (CHO) cells, but they bound to CHO cells transfected with α IIb β 3 genes (A5 cells) and to CHO cells transfected with α v β 3 genes (VNRC3 cells) in a reversible and saturable manner. Scatchard analysis revealed that eristostatin bound to 816000 sites per A5 cell ( K d 28 nM) and to 200000 sites ( K d 14 nM) per VNRC3 cell respectively. However, VNRC3 cells did not bind to immobilized eristostatin. Echistatin bound to 495000 sites ( K d 53 nM) per A5 cell and to 443000 sites ( K d 20 nM) per VNRC3 cell. As determined by flow cytometry, radiobinding assay and adhesion studies, binding of both disintegrins to A5 cells and resting platelets and binding of echistatin to VNRC3 cells resulted in the expression of ligand-induced binding sites (LIBS) on the β 3 subunit. Eristostatin inhibited, more strongly than echistatin, the binding of three monoclonal antibodies: OPG2 (RGD motif dependent), A2A9 (α IIb β 3 complex dependent) and 7E3 (α IIb β 3 and α v β 3 complex dependent) to A5 cells, to resting and to activated platelets and to purified α IIb β 3 . Experiments in which echistatin and eristostatin were used alone or in combination to inhibit the binding of 7E3 and OPG2 antibodies to resting platelets suggested that these two disintegrins bind to different but overlapping sites on α IIb β 3 integrin. Monoclonal antibody LM 609 and echistatin seemed to bind to different sites on α v β 3 integrin. However, echistatin inhibited binding of 7E3 antibody to VNRC3 cells and to purified α v β 3 , suggesting that α v β 3 and α IIb β 3 might share the same epitope to which both echistatin and 7E3 bind. Eristostatin had no effect in these systems, providing further evidence that it binds to a different epitope on α v β 3 .