AbstractMicrospheres made of poly(lactic‐co‐glycolic acid) (PLGA) are biocompatible and biodegradable, rendering them a promising tool in the context of drug delivery. However, nonspecific adsorption of plasma proteins on PLGA micro‐ and nanospheres is a main limitation of drug targeting. Poly(L‐lysine)‐g‐poly(ethylene glycol) (PLL‐g‐PEG), physisorbed on flat metal oxide surfaces, has previously been shown to suppress protein adsorption drastically. The goal of our work was to characterize the efficiency of the protein repellent character of PLL‐g‐PEG on PLGA microspheres and to show the feasibility of introducing functional groups on the PLGA microspheres via functionalized PLL‐g‐PEG. To quantify the adsorbed amount of protein, a semiquantitative method that uses confocal laser scanning microscopy (CLSM) was applied. The first part of the experiment confirms the feasibility of introducing specific functional groups on PLL‐g‐PEG–coated PLGA microspheres. In the second part of the experiment, PLL‐g‐PEG–coated PLGA microspheres show a drastic decrease of adsorbed proteins by two orders of magnitude in comparison to uncoated PLGA microspheres. Low protein‐binding, functionalizable microspheres provide a fundamental basis for the design of drug delivery and biosensor systems. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 66A: 55–61, 2003