Porous silica beads have been modified with aminoalkyl and thioalkyl groups of different chain lengths. The modified silicas were covered with human IgG or human HSA. In order to investigate the influence of the underlying surface on the adsorbed protein layer, the adsorption onto and desorption and displacement of proteins from protein-covered surfaces were studied and compared with those for silica surfaces not previously covered with protein. The underlying surface exerted a great influence on the composition of the protein-covered surface.