We have obtained evidence that selective inhibition of high affinity thrombin-binding sites located in the amino-terminal domain of the membrane glycoprotein (GP) Ib alpha results in impaired platelet activation, as shown by abrogation or reduction of the following responses induced in normal platelets by exposure to less than 1 nM alpha-thrombin: (i) increase in intracellular ionized calcium concentration ([Ca2+]i), (ii) release of dense granule content, (iii) binding of fibrinogen, (iv) aggregation. An anti-GP Ib monoclonal antibody, LJ-Ib 10, which does not inhibit von Willebrand factor binding to platelets, obliterated the high affinity alpha-thrombin-binding sites on normal platelets. Isotherms of alpha-thrombin binding to normal platelets treated with saturating amounts of the antibody were virtually identical to those obtained with platelets from a patient with classical Bernard-Soulier syndrome. In parallel with decreased binding of the agonist, this antibody caused 50% inhibition of the maximal extent of platelet aggregation and 90% inhibition of ATP release induced by 0.3 nM alpha-thrombin. By inhibiting alpha-thrombin binding to GP Ib, the antibody prevented the activation of platelets exposed to low concentrations of the agonist, as demonstrated by abrogation of the increase in intraplatelet ionized calcium concentration induced in control platelets by 0.18 nM alpha-thrombin; under these conditions, fibrinogen binding was inhibited by 84%. Therefore, there is a correlation between occupancy of the high affinity sites for alpha-thrombin on GP Ib alpha and platelet activation, secretion, and aggregation, suggesting that GP Ib alpha is part of an alpha-thrombin receptor relevant for platelet function.