The adsorption kinetics of protein A, BSA, IgG, and fibronectin has been investigated using a homemade quartz crystal microbalance. Information about the energy losses appearing in the system is measured by the maximal oscillation amplitude and the dissipation factor. Only the maximal oscillation amplitude allows us to distinguish the different contributions of liquid and mass to the total frequency shift. The adsorption of proteins has been performed on Ti and Au surfaces at different concentrations. The amount of irreversible adsorbed protein A and IgG increases with increasing bulk concentrations. On Au more proteins adsorb, but their biological activity is reduced in comparison to Ti. Protein A forms a first monolayer in a few seconds, which shows practically no energy losses, and following this a second monolayer is formed. The adsorption rate for the second monolayer is much smaller and energy losses are present. Fibronectin is forming a very viscoelastic system, whose mechanical properties are affected by immersion in different buffer solutions. (C) 2003 Elsevier Science (USA). All rights reserved.