AbstractThe extent and duration of in vivo platelet activation were determined in 12 volunteer donors undergoing automated plasmapheresis. Expression of P‐selectin, activated GpIIb/IIIa, and platelet microparticle formation were measured by flow cytometry on peripheral blood samples obtained immediately before and after plasmapheresis and at 24 hour intervals thereafter for up to 3 days. Although no adverse effects were noted in any donor, immediately after apheresis 3 87% of circulating platelets expressed P‐selectin: by 48 hours. 0.5 50% expressed P‐selectin; and by 72 hours, all donors studied had fewer than 5% P‐selectin expression on circulating platelets. Results were similar for the expression of the activated conformation of GpIIb/IIIa. There was a positive correlation with in vitro P‐selectin expression in response to ADP in the pre‐apheresis sample and the number of platelet microparticles detected in the donor following plasmapheresis. In addition, the percent expression of P‐selectin and activated GpIIb/IIIa in response to ADP was reproducible in each individual studied on five separate occasions (CV ≤ 8%). Platelets activated during plasmapheresis using an automated device may circulate for at least 48 hours, and pre‐plasmapheresis response of platelets to the agonist ADP correlated with platelet microparticle formation post‐plasmapheresis. ©