Polyetheylene glycol (PEG) brushes, for instance, physically tethered to a surface via the adsorbing portion of a PEG-containing copolymer, are a popular protein-resistant surface treatment. Though physisorbed brushes might be displaced by competing species, they continue to dominate diagnostic chips and other applications. In this work, we examine the interactions of two cationic species, poly L-lysine (PLL) and lysozyme, with a popular type of PEG brush, formed by the adsorption of a graft copolymer of PLL PEG on negative silica. Here, 20K molecular weight (MW) PLL comprises the main backbone that adsorbs to the silica and the PEG side chains (2K or SK, in different samples) form tethers. This work examines variations in brush heights and densities, still confining the study to brush architectures (near 30% functionalization of the PLL by PEG side chains) that completely prevent the adsorption of blood proteins such as fibrinogen and albumin. It is found that lysozyme adsorbs to interfaces passivated with these PLL PEG copolymers in amounts that increase with the amount of PEG in the brush. This suggests attractions between the PEG tethers and lysozyme itself. When PLL PEG brushes are challenged by homopolymer PLL (a random coil at the physiological pH studied here), the PLL PEG is almost completely displaced from the silica substrate. The rapid displacement kinetics (with complete loss of protein repellence) for all brush architectures suggest the absence of a steric barrier against PLL penetration of the PEG brush. A small overshoot in surface coverage prior to the displacement of the PLL PEG chains demonstrates the adsorption of PLL on regions of silica at the base of the brush prior to chain displacement, further arguing for the accessibility of the substrate despite the presence of the brush. Differences between the interactions of lysozyme or PLL with the brush suggest an important role of the globular nature of folded proteins compared with random coil polypeptides in protein brush interactions and brush penetration. The results emphasize the technological challenge of retaining seemingly robust brushes adsorbed to interfaces, and eliminating protein adhesion from the brush itself.