We investigate the adsorption processes of a series of positively and negatively charged proteins onto the surface of polyelectrolyte multilayers. We find that proteins strongly interact with the polyelectrolyte film whatever the sign of the charge of both the multilayer and the protein. When the charges of the multilayer and the protein are similar, one usually observes the formation of protein monolayers which can become dense. We also show that when the protein and the multilayer become oppositely charged, the adsorbed amounts are usually larger and the formation of thick protein layers extending up to several times the largest dimension of the protein can be observed. Finally, we find that proteins are mainly adsorbed in a strong way on polyelectrolyte multilayers and protein surface diffusion is strongly suggested. Our results confirm that electrostatic interactions play an important role in polyelectrolyte multilayer/protein interactions.