The protein-protein interactions induced by cation specific effects were characterized by the rotational diffusion analysis. The rotational diffusion coefficient, Drot, estimated by time-resolved fluorescence anisotropy of fluorescent- labeled lysozyme was reduced responding to monovalent cation chlorides and was able to be approximated by quadratic functions of lysozyme concentration. The resultant first and second terms of lysozyme concentration were observed to be negative and positive according to the species and concentrations of monovalent cations, respectively. These two hydrodynamic interaction parameters demonstrated that the attractive and also repulsive interactions were induced between lysozymes in the order of inverse Hofmeister effect.