The release process for adsorbed IgG molecules on titanium particle surfaces in the presence of pure buffer or IgG molecules in solution was investigated by means of radiolabeling techniques. This study is part of a general investigation of the basic mechanisms underlying exchange processes of adsorbed proteins by macromolecules in the bulk and in particular of the kinetic laws describing such processes. This study shows the existence, in the investigated time scale (from 1 to 43 h), of two different populations of adsorbed IgG molecules : a population called type I which Is releasable from the surface and a type II population which is irreversibly adsorbed on the surface. As in previous studies, we confirm that for the population of type I, the kinetics of the release mechanism is enhanced by the presence of Ige molecules in solution. This is a strong indication that it constitutes an exchange process. Moreover, we show that this release process is of order 1 with respect to both adsorbed and bulk molecules. Finally, Re show that the percentage of molecules of type I (reversibly adsorbed on the surface at the adsorption time scale investigated) seems to increase linearly with the bulk concentration of proteins during the release experiments, in the investigated bulk concentration range. This unexpected result constitutes a new observation and we do not posses a satisfactory explanation for it.,A similar increase was also reported for exchange experiments performed with polyacrylamide onto aluminosilicated glass beads by Pefferkorn et al.