We have studied the ability of the plasma to induce aggregation of both homologous and heterologous platelets in four patients with hemolytic-uremic syndrome (HUS) associated with chemotherapy with mitomycin C (MMC). Neither platelet aggregation was elicited by patients' plasmas nor the in vitro addition of purified von Willebrand factor (vWF) had any effect on the aggregation pattern. In addition, ristocetin-induced binding of patients' vWF to formaldehyde-fixed platelets was normal, and multimeric vWF analysis revealed a normal structure of patients' plasmatic vWF whatever the clinical stage in which it was studied. These findings suggest that, in spite of the existence of common clinical and biological features in the various forms of HUS, the pathogenesis of MMC-related HUS may be, at least in part, different from that of the other forms of HUS in which both platelet-aggregating activity and alterations in the vWF are found.