Platelet immunophenotyping was performed on whole blood with monoclonal antibodies against GPIIb-IIIa (LYP18), GPIbα (SZ2), GPIX (SZ1), and GPV (SW16). Samples were analyzed for fluorescent intensity by flow cytometry. Fluorescent intensity was translated into sites per platelet by using a calibration curve generated from beads coated with known densities of IgG molecules. The data represent mean ± SEM values in blood samples from 3 healthy donors (controls) or samples obtained from the BSS patient on 3 different days. Values for GPIb and GPIX density on platelets from the BSS patient were at the threshold of detection but significantly higher than background levels. Platelet size was determined by measuring the maximal diameter of platelets stained with Alexa 488–LYP18, spread on fibrinogen-coated glass slides, and imaged on a confocal fluorescent microscope. Platelet height was the depth of the adhered platelets calculated from z-sections. All the platelets in a microscopical field were measured to avoid selection bias. Data represent mean ± SEM values for 40 individual platelets obtained from 3 healthy donors or from the BSS patient on 3 different days.