Type 2B von Willerbrand's disease (VWD) is a variant in which the structurally abnormal von Willebrand factor (VWF) has an increased affinity for the platelet glycoprotein Ib-IX-V complex. Spontaneous binding of type 2B VWF to platelets and their subsequent clearance from the plasma appear to account for the characteristic phenotype of type 2B VWD. Several type 2B mutations have been described and shown to be grouped along the amino edge of the beta sheet of the VWF A1 domain. In this report we describe a novel missense, Arg534Leu, in the VWF A1 domain in three members of a family with type 2B VWD. We have expressed and characterized the corresponding recombinant mutant VWF in transiently transfected COS-7 cells. Relative to wild-type WVF, recombinant Arg543Leu VWF showed a similar multimeter composition but exhibited binding to fixed platelets both in the presence and absence of ristocetin, confirming the ability of this mutation to permit spontaneous interaction of VWF with platelets. These studies are consistent with a recently proposed model in which the VWF-A1 domain exists in either 'on' or 'off' states, with type 2B mutations switching VWF to an 'on' state to facilitate GPIb binding.