Apoptosis is triggered during 37°C storage, but inhibition of apoptosis does not improve platelet viability. Murine platelets were stored at 37°C with gentle agitation in 5% plasma for up to 48 hours. (A) Caspase-3/7 activation, (B) annexin V-positive platelets, (C) ATP activity (CellTiter-Glo assay), (D) TMRM-positive platelets, (E) GPIbα surface expression, and (F) P-selectin exposure were assessed at indicated points by flow cytometry or as otherwise indicated. Platelet counts were adjusted before storage. Data are presented as mean ± SEM. Caspase activity is presented as mean ratio/start for each genotype. n = 8 to 10 biological replicates per genotype. Statistical significance between treatment groups was analyzed using an unpaired Student t test with 2-tailed P values. *P < .05; ***P < .001.