The alpha IIb beta 3 integrin binds Arg-Gly-Asp-containing (RGD-containing) ligands in a cation-dependent interaction. A fourteen amino acid sequence, beta 3 (118-131), and an antibody to it, inhibited ligand binding functions of alpha IIb beta 3, and a 1:1 stoichiometric beta 3 (118-131)-RGD complex was detected by mass spectroscopy. Cation binding to beta 3 (118-131) was demonstrated by terbium luminescence and mass spectroscopy. Notably, ligand displaced cation from the beta 3(118-131) peptide and also from purified alpha IIb beta 3. Thus, beta 3 (118-131), a highly conserved region in integrin beta subunits, binds both ligand and cation. Formation of a ternary complex between cation, ligand, and receptor, with subsequent displacement of cation from beta 3 (118-131) and a second site within the receptor, may be central to the mechanism of ligand recognition by integrins.