A ligand derived from, e.g. a Fab fragment of, a monoclonal antibody obtainable from the cell line deposited with the Belgian Coordinated Collections of Micro-organisms under accession number LMBP 5108CB binds to the human platelet glycoprotein GPib and prevents the binding of von Willebrand factor to said GPIb without inducing thrombocytopenia. The said ligand is useful, in admixture with a pharmaceutically acceptable carrier, in a pharmaceutical composition, optionally further comprising a thrombolytic agent, for preventing and/or treating haemostasis disorders.