Human coagulation Factors VII and VIIa bind with equal affinity to monocytes stimulated with endotoxin. Equilibrium binding studies performed at 0°C using 125I-labeled Factor VII and VIIa showed the dissociation constant (K(d)) to be ≃82 pM with ≃3,600 binding sites/monocyte. Ca++ was required for Factor VII and VIIa interaction with monocytes (optimal CaCl2 concentration ≥2.5 mM) and binding was reversed by the addition of EDTA. The rate of conversion of Factor X to Xa in mixtures containing Factor VIIa and monocytes was directly related to the quality of Factor VIIa bound to the monocyte surface. Thus the monocyte binding sites appear to represent tissue factor. Competition experiments showed that Factor VII and VIIa bind to the same monocyte sites and further, that unlabeled Factor VII and VIIa have the same affinity for the binding sites as the 125I-labeled proteins.