Life-threatening anaphylaxis or febrile nonhemolytic transfusion reactions after transfusion of platelet concentrates (PCs) is a serious clinical problem caused by the sensitizing of recipients to plasma components, such as immunoglobulin A, or by cytokines. There is a possible indication for washing of PCs in these thrombocytopenic patients. However, only platelets that show activation after physiological stimulation are useful. We determined the spontaneous and induced activation of platelets before and after washing. We investigated 11 consecutive single-donor-apheresis PCs. After production and leukocyte-depletion the PCs were washed in 15%, acid-citrate-dextrose-solution. The spontaneous and the adenosine diphosphate (ADP)-induced, as well as collagen-induced activation, were determined by flow cytometry. Additionally, ADP- and collagen-induced aggregation were measured. Unwashed platelets (16.1%) were activated spontaneously. The washing of PCs led to a threefold increase of spontaneous activation of platelets (47.4%). Because of increased spontaneous activation after washing we could demonstrate cytometrically a loss of induced activation of washed platelets. Furthermore, washing resulted in an impaired ADP-induced aggregability of platelets. These results have led us to reduce the frequency of washing of PCs in our institution, where the only current indication for washing of PCs is in patients with a history of severe nonhemolytic transfusion reactions.