Changes in ligand binding ability of the integrin alpha IIb beta 3 can be monitored by the concomitant expression of ligand-inducible binding sites (LIBS). A new LIBS, the hexapeptide sequence GPNICT (residues 1-6) at the amino terminus of beta 3 recognized by the murine monoclonal antibody (mAb) AP5, is sensitive both to the binding of ligand and to micromolar differences in divalent cation levels. Calcium or magnesium can completely inhibit the binding of AP5 to alpha IIb beta 3 on platelets, with ID50 values of 80 and 1500 microM, respectively. The inhibitory effect of calcium plus magnesium is cumulative. In the presence of 1 mM calcium plus 1 mM magnesium, the peptide RGDW overcomes this inhibition and induces maximal binding of AP5. Maximal AP5 binding is also induced by a molar excess of EDTA. The unique location of the AP5 LIBS was determined by comparing the binding of LIBS-specific mAb to recombinant human-Xenopus beta 3 chimeras produced in a baculovirus expression system. AP5 defines one region at the amino terminus beta 3 1-6. A second region, defined by mAb D3GP3, is probably located within beta 3 422-490, confirming the finding of Kouns et al. (Kouns, W. C., Newman, P.J., Puckett, K. J., Miller, A. A., Wall, C. D., Fox, C. F., Seyer, J. M., and Jennings, L. K. (1991) Blood 78, 3215-3223). The third region, encompassing at most residues 490-690, and perhaps more precisely located within 602-690 (Du X., Gu, M., Weise, J. W., Nagaswami, C., Bennett, J. S., Bowditch, R., and Ginsberg, M. H. (1993) J. Biol. Chem. 268, 23087-23092), is recognized by the four mAb, anti-LIBS2, anti-LIBS3, anti-LIBS6, and P41. Since its exposure is uniquely regulated by both divalent cations and ligand, the amino terminus of beta 3 may be involved in control of ligand binding by divalent cation mobilization.