The effect of surface charge on the protein resistance of adsorbed layers of poly(ethylene imine)-[g]-poly(ethylene glycol), PEI-PEG, and poly(L-lysine)-[g]-poly(ethylene glycol), PLL-PEG, was studied. Mixed and monofunctional self-assembled monolayers, SAMs, on gold were obtained by adsorption of 16-mercapto-1-hexadecanoic acid and 16-mercapto-1-hexadecanol. The surface charge was systematically varied by changing the ratio of the two alkanethiols. The graft copolymers PEI-PEG and PLL-PEG were adsorbed at the SAMs and tested for resistance towards human serum albumin and fibrinogen. The adsorbed amount of copolymers increased with increasing negative surface charge. However, the best protein resistance was found at an intermediate surface charge. The PLL-PEG covered surfaces showed better protein resistance than the PEI-PEG covered surfaces. Thus, this work demonstrates that an adsorbed layer of PEG-grafted PEI and, in particular, PEG-grafted PLL is efficient in preventing protein adsorption when there is charge neutralization between the copolymer and the underlying surface.