As one of the most important acute-phase reactants in human serum, C-reactive protein plays its physiological roles mainly on membranes. Here we show that the human C-reactive protein is two-dimensionally crystallized upon specific adsorption on the phosphorylcholine ligand containing membranes by monolayer approach. The 2.0-nm resolution projection structure of the two-dimensional crystals analyzed by electron microscopy and image reconstruction reveals open-ring-like pentamers in the crystals. The electron microscope graphs also show that the dissociated pentamers with open-ring-like structure occur in a closed packing region (not two-dimensionally crystallized). These results indicate a membrane-induced dissociation and rearrangement of hCRP, which may relate to the variety of hCRP's physiological functions.