Storage of single-donor platelet concentrates is currently limited to 5 days. During this period, however, numerous morphologic and biochemical changes have been observed. These changes result in functional impairment of stored platelets. The present study describes increased binding of a monoclonal antibody against GMP 140 on the surface of stored single-donor platelets revealing an activation process. In contrast, binding of monoclonal antibodies directed against glycoprotein complex (GP) IIb-IIIa and ligand-induced binding site (LIBS1) is slightly diminished during storage. When platelets are stimulated with ADP GMP 140, GP IIb-IIIa, and LIBS1 are expressed to a higher extent than on the surface on nonstimulated platelets. The quantity exposed, however, depends upon the storage time. It is significantly reduced when platelets are stored for longer than 1-2 days. The present data indicate that storage of single-donor platelet concentrates affects fibrinogen binding, cell to cell cohesion, and release reaction. The results are in good agreement with conventional aggregation and in vitro bleeding time measurements.