von Willebrand factor (vWF) is a multimeric glycoprotein composed of multiple homologous domains. The A1 domain contains a remarkably large disulfide loop of 185 amino acids (Cys 509-695) which plays a key role in promoting platelet adhesion to the subendothelium. Following the initial binding of the A1 domain to the subendothelium, a conformational change occurs which allows its binding to platelet GPIb. In an attempt to further understand the structure-function relationship of the A1 domain, we analyzed 1) the functional properties of recombinant vWF mutated on either Cys 509 or 695 and 2) the reactivity of a monoclonal antibody (MoAb B724) with vWF conformations interacting with GPIb. Our data underline the crucial role of the 509-695 disulfide bond in the binding of vWF to GPIb and discriminate the specificity of each Cys in this binding. They also indicate that two different conformations of the A1 loop, with high or low affinity for MoAb B724, allow the exposure of its GPIb-binding site. Since the low affinity conformation is observed in type 2B vWF, MoAb B724 appears as a useful tool to probe this type of von Willebrand disease (vWD).