A platelet-agglutinating factor has been detected in the plasma of some patients during episodes of thrombotic thrombocytopenic purpura (TTP).^{1 , 2} Agglutination induced in vitro by this plasma factor is inhibited by normal plasma and does not require platelet production of the arachidonic acid metabolite thromboxane A₂, release of granule contents, platelet metabolism, or calcium or magnesium ions.³ These characteristics are similar to those of agglutination induced in vitro by the attachment to platelets of large, multimeric, von Willebrand factor components of the factor VIII complex (VIII:vWF), and this similarity led us in the present study to investigate the properties . . .