To measure platelet activation, platelets were isolated from healthy donors, pretreated with 10–20 µM of tamoxifen or vehicle control, washed, and exposed to agonists (ADP, MCF-7 tumor cells or TRAP). (A). P-selectin surface expression, a marker of activation, was determined by flow cytometry. Representative histograms are shown, with P-selectin stained platelets (gray) overlaid onto platelets stained with isotype control antibodies (red) (B). Quantified results are shown in C. Bars indicate SEM. P<*0.01 compared to resting control unless otherwise indicated by ANOVA, n=3–6 independent replicates per treatment group.