Trifluoroethanol has often been used in protein folding studies.The changes in activity and unfolding of arginine kinase from shrimp Feneropenaeus chinensis muscle during denaturation in different concentrations of trifuoroethanol were investigated using far-ultraviolet circular dichroism and fluorescence emission spectra.Arginine kinase was inactivated in trifluoroethanol solutions.The tertiary and secondary structures of arginine kinase were also destroyed in the trifluoroethanol solutions.The unfolding and inactivation courses were measured and compared.Inactivation occurred prior to unfolding, which suggests that the arginine kinase active site is more easily damaged by the denaturant than the enzyme as a whole.The result also indicates that the arginine kinase active site is situated in a limited and flexible region of the enzyme molecule.