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This study is a systematic attempt to understand the roles of osmolytes in protecting proteins against denaturing stress. Thermal denaturation of cytochrome c has been studied in the presence of various concentrations of all L-amino acids that are more hydrophobic than glycine and have a solubility of 0.1 M or higher in water at 25 degrees C. The basic observations are as follows. (1) Arginine and histidine destabilize the native protein; both Tm (the midpoint of thermal transition) and delta GDH2O (25 degrees C) (the Gibbs energy of stabilization) decrease with increasing amino acid concentration. (2) Isoleucine, leucine and phenylalanine have no effect on Tm and deltaGDH2O (25 degrees C). (3) Valine and less hydrophobic amino acids stabilize the protein in terms of Tm but deltaGDH2O (25 degrees C) is unchanged. This observation was confirmed by the study of isothermal denaturation of cytochrome c by …