Terminally grafted chains of poly(ethylene glycol) (PEG) and oligo(ethylene glycol) reduce protein adsorption and cell adhesion on material surfaces. However, previous studies showed that protein-PEG adhesion is induced by the application of pressure. Because polymer behavior can vary with the molecular weight, in this study we directly measured the forces between streptavidin and end-grafted monolayers of PEG of different molecular weights and at grafting densities. The results of these measurements show that grafted PEG chains can exist in two different states: a protein-repulsive state and a protein-attractive state. The attractive state can be induced not only by compression but also by increasing the temperature or by altering the polymer molecular weight. Both the critical applied load to induce the protein-attractive form of PEG and the relaxation time back to the protein-resistant state depend on the molecular weight of the grafted chains. The consequences of the observed behavior for the use of grafted PEG chains as protein antifouling coatings of biomaterials are discussed.