Radioiodinated Bolton-Hunter reagent was used at low specific activity to probe for the function and reactivity of amino groups on von Willebrand factor (vWF), a plasma protein involved in platelet responses to damaged endothelial surfaces. The platelet receptor for vWF contains a membrane protein termed glycoprotein Ib. Modification of only one or two amino groups per vWF subunit caused a 50% reduction in the platelet-agglutinating activity of vWF, and a decrease in its ability to bind to platelets. All multimeric forms of vWF are modified. Loss of platelet-agglutinating activity on modification of less than 2% of the amino groups on each vWF subunit suggests that the amino groups in the glycoprotein Ib-binding domain of vWF are both particularly reactive and essential for its function.