The competitive adsorption of fibrinogen and high molecular weight kininogen at different flow rates on silica capillaries has been studied at 37 degrees C in Tris buffer (Tris 0.05 M; NaCl 0.10 M) and at a dilution of 10(-2) with respect to the plasma concentrations. By radiolabeling (I-125 and I-131) each protein differently, it is possible to follow the adsorption of both molecules from the mixture simultaneously. An accumulation of both proteins at the interface followed by a progressive release of fibrinogen was observed. This release was not necessarily associated with an increase of the kininogen interfacial concentration. From an analysis of the initial kinetics of adsorption of kininogen vs wall shear rate, the diffusion coefficient (D approximate to 4.4 x 10(-7) cm(2) s(-1)) and the adsorption constant (K-a approximate to 2.4 x 10(-4) cm s(-1)) have been derived. The final adsorption kinetics of kininogen in the presence of fibrinogen is more compatible with a site adsorption model than with a random sequential adsorption model and leads to a much smaller adsorption constant, of the order of 10(-5) cm s(-1). This suggests that kininogen does not interact directly with the bare silica but; with fibrinogen molecules, which act as adsorption sites. For the molecular ratio of proteins in solution used here, it was found that the interfacial concentrations of both proteins are independent of shear rates at the maximum of fibrinogen concentration (0.4 mu g cm(-2) for fibrinogen (Fib) and 0.1 mu g cm(-2) for kininogen (HK)), although appearing at varying times. These concentrations correspond to a molecular ratio HK/Fib of 0.9, which is close to an equimolecular composition at the interface. However at the beginning of the adsorption process, we measured one molecule of kininogen for 10 molecules of fibrinogen at the surface, in accordance with a theoretical estimation from the solution concentrations and molecular masses.