The heterogeneous exchange process whereby IgG molecules adsorbed on a hydrophilic silica surface are replaced by fibrinogen molecules from the solution has been investigated by means of I-125 radiolabeling. Two characteristic times appear both in the desorption and replacement processes. At short times (on the order of 1 h), the process seems to correspond to an exchange mechanism which can be modeled by a first-order kinetic reaction with respect to the fibrinogen molecules. Furthermore, it also appears to be of first-order with respect to the adsorbed IgG molecules. The exchange rate constant is equal to (3.84 +/- 0.05) x 10(5) L mol(-1) h(-1). This generalizes previous conclusions drawn from homogeneous exchange studies both on polymer and protein systems. The long term behavior of the replacement process is characterized by a time scale which is identical to that of the desorption process. It thus seems to correspond to a desorption/readsorption process.