A new model for biospecific sorption of proteins is proposed. The model considers adsorption on a rigid non-swelling matrix, in particular, silica. The quantity of ligand accessible to a protein is estimated using the fractal approach. Steric (not allosteric) interactions between a protein sorbed and a cluster of attached ligands are discussed. It is shown that biospecific binding of a protein decreases as the surface concentration of attached ligands is increased.