C3 and C9 binding and CD40L and CD62P expression on normal platelets exposed to serum. Washed normal platelets (from citrated tubes) were incubated with an equal volume (25 μL) serum from patients 1 (♦), 2 (▴), 3 (●), 4 (◇), parents, or controls (○). The patients' serum exhibited increased C3 and C9 binding to normal platelets compared with control sera. Likewise, patient sera resulted in increased CD40L and CD62P expression, suggesting activation. This was not seen when platelets were exposed to normal sera. Serum from the parent bearing the same mutation as the affected patient with aHUS exhibited similar results to the patients, whereas sera from the parents that did not bear the mutation resembled normal sera with the exception of the father of patient 1 (treated at the time of sampling for lymphosarcoma). Results for patients were carried out 3 times, and for the parents and controls twice with reproducible results. *P < .001, when comparing patient sera with control sera. Results are expressed as median plus or minus SD.