The PFA-100 platelet function analyser simulates high-shear platelet function; however, the relative importance of the high shear rates alone has not been examined in this system. Normal blood was analysed by the PFA-100 using modified cartridges that contained a blank membrane lacking the usual platelet agonists (collagen/epinephrine or collagen/ADP) thus isolating the effects of shear stress alone. We found evidence of platelet activation with significant increases in platelet–leucocyte aggregates. There was no significant change in surface CD62P nor PAC-1 expression. A degree of platelet activation occurs in the PFA-100 due to shear stress alone.