LS174T, COLO205 cells and neutrophils (106/mL) were perfused over a platelet P-selectin substrate (αIIbβ3-blocked platelets) at wall shear stresses of 0.8 dyn/cm2 (LS174T and COLO205 cells) and 1.0 dyn/cm2 (neutrophils). The critical velocity of free-flowing tumor cells adjacent to the wall was experimentally calculated to be about 700 μm/s.42