Washed human platelets in buffers containing either 2 mM Ca⁺⁺ or 4 mM EDTA were stimulated by human α-thrombin to induce secretion. The binding of two endogenous secreted proteins, factor-VIII-related protein (Vlll-R) (von Willebrand factor) and platelet factor 4, was measured by reacting thrombin-treated and control platelets with specific antibodies to these proteins, then quantifying antibody binding with ¹²⁵l-staphylococcal protein A. Both of these granule proteins were associated with the platelet membrane surface by a calcium-dependent mechanism after thrombin-induced secretion. This ability to bind endogenous secreted proteins to the plasma membrane surface may provide a mechanism by which the platelet can concentrate and organize its secreted proteins for subsequent physiologic reactions.