The glycoprotein (GP) Ib-IX complex is a platelet surface receptor that binds thrombin as one of its ligands, although the biological significance of thrombin interaction remains unclear. In this study we have used several approaches to investigate the GPIbα-thrombin interaction in more detail and to study its effect on the thrombin-induced elaboration of fibrin. We found that both glycocalicin and the amino-terminal fragment of GPIbα reduced the release of fibrin-opeptide A from fibrinogen by about 50% by a noncompetitive allosteric mechanism. Similarly, GPIbα caused in thrombin an allosteric reduction in the rate of turnover of the small peptide substrate D-Phe-Pro-Arg-pNA. The Kd for the glycocalicin-thrombin interaction was 1 μM at physiological ionic strength but was highly salt-dependent, decreasing to 0.19 μM at 100 mM NaCl (Γsalt = -4.2). The salt dependence was characteristic of other thrombin ligands that bind to exosite II of this enzyme, and we confirmed this as the GPIbμ-binding site on thrombin by using thrombin mutants and by competition binding studies. R68E or R70E mutations in exosite I of thrombin had little effect on its interaction with GPIbα. Both the allosteric inhibition of fibrinogen turnover caused by GPIbα binding to these mutants, and the Kd values for their interactions with GPIbα were similar to those of wild-type thrombin. In contrast, R89E and K248E mutations in exosite II of thrombin markedly increased the K d values for the interactions of these thrombin mutants with GPIbα by 10- and 25-fold, respectively. Finally, we demonstrated that low molecular weight heparin (which binds to thrombin exosite II) but not hirugen (residues 54-65 of hirudin, which binds to exosite I of thrombin) inhibited thrombin binding to GPIbα. These data demonstrate that GPIbα binds to thrombin exosite II and in so doing causes a conformational change in the active site of thrombin by an allosteric mechanism that alters the accessibility of both its natural substrate, fibrinogen, and the small peptidyl substrate D-Phe-Pro-Arg-pNA.