Blood coagulation is initiated after blood is exposed to the transmembrane glycoprotein Tissue Factor (TF). The continuous flow reactor has been used to study the flow-dependent activation of factor X by the complex of TF and factor VIIa. The apparent Km value of X for TF-VIIa in vesicles of 16 nM is low compared to the value of > 600 nM on a macroscopic surface. At 17 to 70 fmoles TF-VIIa/cm2, the steady state Xa production rate was fairly insensitive to the enzyme density, which is in contrast with classical Michaelis-Menten kinetics. The X activation rate was linear with the cube root of the flow rate. These results show that X activation is limited by transport of factor X across a boundary layer towards the enzyme complex.