The activation of platelets during the preparation of platelet concentrates (PCs) by two methods was compared. To eliminate interdonor differences, 2 units of whole blood were pooled and subsequently divided into two batches. From one batch, the platelets were harvested as pelleted platelets from platelet-rich plasma (PRP) and from the other as nonpelleted platelets from the buffy coat (BC). The activation of platelets in these PCs was studied immediately after preparation and during storage for up to 9 days at 22 degrees C with gentle agitation. The binding of monoclonal antibodies (MoAbs) against the GP IIb/IIIa complex and against activation-dependent antigens (GMP 140 from the alpha granules and a 53-kDa glycoprotein from the lysosomal granules) was measured. Beta-thromboglobulin (beta-TG) release was also determined. Disc-to-sphere transformation was quantitated by measuring on an aggregometer the difference in light transmission during stirring at different rates and also by light microscopy. Immediately after preparation, platelets derived from PRP had a more spheric morphology (p less than 0.01), had a higher beta-TG release (p less than 0.01), bound more MoAbs against GP IIb/IIIa (p less than 0.01), and expressed more GMP 140 and 53-kDa glycoprotein (p less than 0.01) than did BC-derived platelets. However, these differences had disappeared after 2 days of storage. It was concluded that, immediately after preparation, PRP-derived platelets are more activated than BC-derived platelets. This is most likely a result of the pelleting that follows the second high-speed centrifugation of the PRP.