Flow cytometric analysis of patients' and normal platelets. Analysis was performed on whole blood with monoclonal antibodies against GPIbα (AP1) and GPIIbIIIa (AP2). Platelets were then analyzed for their binding to the anti-GPIbα antibodies AP1 (solid bold line), 142.2 (solid line), and 142.11 (dashed line), the anti-GPIX antibody (hatched area), and an irrelevant mouse IgG (gray area). The anti-GPIbα antibodies bind to normal platelets, but not to those in this patient. There are similar amounts of GPIX on the surface of platelets from the patient and a normal volunteer.