C-reactive protein (CRP) is one of the most characteristic acute-phase proteins. Modified CRP is the monomeric form of native CRP and has recently been suggested to exist under physiological conditions. In the current work, CRP subunits were separated from stock CRP solution by size-exclusion chromatography. Two forms of two-dimensional crystals composed of monomeric CRP were obtained on negatively charged lipid monolayers: a previously reported form, MI, and a new form, MII. A projection map at 2.0 nm resolution of the two-dimensional MII crystals was obtained. The formation of the two forms of two-dimensional crystal exhibited a dependence on pH. At pH values of less than 5.5 the subunits assembled in MI packing, while at pH values greater than 6.5 they assembled in MII packing. When using modified CRP prepared by acidic denaturation, only MI crystals could be formed. The fact that CRP subunits produced by dissociation or denaturation could form highly ordered two-dimensional crystals indicates that they have a certain homogeneous structure, which supports the previous suggestion of the existence of modified CRP in vivo.