Membrane glycoprotein (GP) expression on platelets changes during preparation of concentrates and storage. Recent studies proved that the increased expression of the GMP-140 (CD 62), a GP appearing on platelets after activation, correlates with the in vivo recovery of transfused platelets. We therefore investigated the expression of this and three other GP (GP IIb/IIIa, GP, Ib, GP 53), detected by flow cytometry, in normal controls and in platelet concentrates prepared by four different methods (apheresis, preparation from buffy coat, from pooled buffy coat and platelet-rich plasma) after preparation and during storage. Furthermore we investigated the influence of filtration, radiation and washing platelet concentrates on the expression of GP. The results can be summarized as follows: (1) The mean surface fluorescence (MF) is a better parameter for the evaluation of preparation and storage lesions than the total number of positive cells. (2) The mean fluorescence of GP Ib in platelet concentrates is generally lower than in normal controls and decreases continuously during storage; there is a small loss of GP Ib through washing and filtration. (3) GP IIb/IIIa fluctuates during preparation and storage, therefore it is not suitable for evaluation of quality. (4) The percentage of CD 62 positive cells and their mean fluorescence is higher in platelet concentrates than in normal controls and increases during storage continuously. There is a weak correlation of CD 62 with CD 63. (5) The changes of GP in apheresis concentrates and concentrates obtained from whole blood are similar after preparation and during storage.