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Increases in plasma concentrations of platelet granule products such as platelet factor 4 and β-thromboglobulin during cardiopulmonary bypass suggest that platelets are activated during extracorporeal circulation. Subsequent circulation of these activated platelets may be responsible for the ubiquitous platelet dysfunction associated with cardiopulmonary bypass. Using flow cytometry and a monoclonal antibody directed against an α-granule membrane protein, granule membrane protein 140 (GMP-140), which is expressed on the platelet surface membrane after activation, we directly measured the percentage of circulating activated platelets in 41 patients before, during, and after cardiopulmonary bypass. In addition, we compared the GMP-140 expression with platelet aggregation in response to adenosine diphosphate (ADP). Cardiopulmonary bypass produced a significant increase in the percentage of GMP-140-positive platelets persisting in the circulation; the percentage peaked at a mean of 29% (range 10–58%) before separation from extracorporeal circulation. A significant percentage of these activated platelets continued to circulate in the early postoperative period. Simultaneous measurement of platelet aggregation in response to ADP demonstrated an aggregation defect that had a time course distinct from platelet activation and whose magnitude did not correlate with the degree of platelet activation in individual patients. We conclude that cardiopulmonary bypass causes a complex constellation of platelet defects, which include α-granule release, prolonged circulation of activated, “spent” platelets, and impaired platelet aggregation.