BACKGROUND The use of fresh platelets results in better posttransfusion recovery and survival than does the use of platelets that have been stored before transfusion. Activation of platelets during preparation and storage may be one of the factors responsible for a number of storage-related changes in platelet membrane proteins. Blood centers commonly prepare platelet concentrates from both multiple units of whole blood and single-donor plateletpheresis collections. METHODS Seventeen plateletpheresis concentrates, anticoagulated with ACD, were compared to platelets prepared from whole blood from the same donor that was anticoagulated with CPDA-1 (random-donor platelets). After preparation, plateletpheresis and random-donor platelets were stored in plastic storage bags at 22 degrees C for 5 days. Platelet surface glycoproteins were examined by flow cytometry after platelets were fixed in dilute plasma with 1-percent formaldehyde and stained with fluorescein isothiocyanate-labeled monoclonal antibodies CD42b (anti-glycoprotein [GP]lb), CD41a (anti-GPllb/llla), and CD62 (anti-P-selectin). RESULTS The binding of anti-CD42b was greater in plateletpheresis concentrates than in random-donor platelets on Days 3 and 5 (p < 0.01) of storage; binding of anti-CD62 was greater in the random-donor concentrates (p < 0.01) on Days 3 and 5. Plateletpheresis concentrate aggregation responses were greater on Day 5 (p < 0.01). To determine if the type of anticoagulant and the method of mixture with blood contributed to these changes, 10 samples were split into aliquots and prepared in two separate ways: One group of samples was prepared by allowing anticoagulant (ACD) and blood to flow into the tube at a rate of 3 microL per second, and the other group of samples was prepared by allowing blood to flow into tubes containing a measured amount of CPDA-1. The first samples bound more anti-CD42b than the second samples (p < 0.01). The second group of samples contained significantly more microvesicles that bound anti-CD41a than did the first group (p < 0.01). Samples prepared by the first method but anticoagulated with CPDA-1 contained more microvesicles but had the same amount of anti-CD42b binding as did similarly prepared samples anticoagulated with ACD (p < 0.05). CONCLUSIONS Platelet concentrates prepared from single units of whole blood and anticoagulated with CPDA-1 bind less anti-CD42b and more anti-CD62 than do platelets obtained by apheresis. These differences may be attributed to platelet sedimentation and the transient exposure of some of the platelets in the blood that is first collected during whole-blood donation to high concentrations of anticoagulant.