We have studied the structure of salt-free lysozyme at 293 K and pH 7.8 using molecular simulations and experimental SAXS effective potentials between proteins at three volume fractions, phi = 0.012, 0.033, and 0.12. We found that the structure of lysozyme near physiological conditions strongly depends on the volume fraction of proteins. The studied lysozyme solutions are dominated by monomers only for phi <= 0.012; for the strong dilution 70% of proteins are in a form of monomers. For phi = 0.033 only 20% of proteins do not belong to a cluster. The clusters are mainly elongated. For phi = 0.12 almost no individual particles exits, and branched, irregular clusters of large extent appear. Our simulation study provides new insight into the formation of equilibrium clusters in charged protein solutions near physiological conditions. (C) 2011 Elsevier Inc. All rights reserved.