Low-protein-fouling poly(ethylene glycol) (PEG-like) plasma polymer films were prepared using radio frequency glow discharge polymerization of diethylene glycol dimethyl ether (DGpp) on top of a heptylamine plasma polymer primer layer. By varying the plasma deposition conditions, the chemistry of the DGpp film was influenced, especially in regard to the level of ether content, which in turn influenced the relative levels of bovine serum albumin and lysozyme protein fouling. Surface potential measurements indicated that these surfaces carried a net negative charge. While protein fouling remained low ( approximately 10 ng/cm2), there was a slightly higher level of the positively charged protein adsorbed on these films than the negative protein. The interaction forces measured between a silica spherical surface on both "high"- and "low"-protein-fouling DGpp films were all repulsive and short ranged (2-3 nm). There was no correlation between the surface forces measured for high- and low-protein-fouling DGpp films. Thus, it appears that enthalpic effects are very important in reducing protein adsorption. We therefore conclude that it is the concentration of residual, ethylene glycol containing species that are the crucial parameter determining protein resistance due to a combination of both entropic and enthalpic effects.