The hydrodynamic properties of the C-reactive protein (CRP) at different pH were studied using quasi-elastic light scattering, size-exclusion liquid chromatography, and nonreducing gel electrophoresis. It was shown that a CRP solution at pH 5.0-7.2 presents a polydisperse system the major component of which is the native pentameric CRP. At pH 4.0-4.5, CRP exists in two states having different hydrodynamic properties: the native pentameric form with a molecular mass of 120 kDa and with the hydrodynamic radius of 4.03 nm and high-molecular-weight aggregates with a wide range of their molecular weight distribution. The interaction of the C-reactive protein with monoclonal antibodies to it indicates that conformation-dependent surface epitopes of the protein lose the native structure at pH 5.0-5.5. The aggregation of CRP is an irreversible process, which begins in a narrow pH range of pH 5.0-4.5 and is not accompanied by the dissociation into subunits but is determined by intermolecular interactions of its quasi-native pentamers.