Studies of surface-labeled platelets and platelet releasate. Platelets were washed (PBS-EDTA-theophylline) and surface-labeled with biotin. In parallel experiments, the releasate from thrombin-activated platelets was labeled with biotin. Aliquots (10 μL) of the labeled releasates, solubilized surface-labeled platelets (total lysate), and the corresponding membrane fractions (solubilized, washed pellets from freeze-thaw lysates) of surface-labeled platelets were analyzed using nonreduced/reduced SDS-PAGE. Biotinylated proteins were detected using streptavidin-peroxidase and chemiluminescent substrate. Platelet GPs Ib, α_IIbβ₃ , α₂β₁ , and thrombospondin (normal thrombospondin []; degraded thrombospondin [➭]) are indicated. Samples from 2 patients and a control (C), processed in parallel, are shown.