In this study, the results of protein adsorption on a wide spectrum of different surface-attached polymer networks are presented. The networks were generated from copolymers containing photoactive benzophenone groups or thermally activated azosulfonyl groups. The copolymers were deposited as thin films onto solid substrates and then (photo-)activated, which leads to crosslinking and the formation of surface-attached polymer networks. This simple preparation protocol allows the study of a wide range of polymer networks with well-defined hydrophilicity and/or charge and permits thus control of the degree of swelling in a contacting aqueous buffer solution. The so obtained polymer surfaces were exposed to solutions containing fibrinogen or other proteins and the amount of protein adsorbed was measured by surface plasmon resonance spectroscopy (SPR). It was found that strongly swollen polymer systems, which have only weak enthalpic interactions with the proteins, show strong protein repellency irrespective of the details of the chemical composition. We propose that conformational changes in the surface-attached polymer networks ("entropic shielding") and/or size exclusion effects dominate the adsorption behavior. In addition, we show first results of cell adhesion experiments of endothelial cells on these networks. It was found that surfaces which are protein resistant show also strong cell repellency, so that the surface-attached polymer networks might be used as a valuable tool to control cell adhesion.