Normal modes of low-frequency vibrations are calculated for a small globular protein, bovine pancreatic trypsin inhibitor. In modes with frequencies below 120 cm-' the protein molecule behaves like a continuous elastic body. Most modes with frequencies above 50 cm-' are shown to behave harmonically within the range of thermal fluctuations at room temperature. Those with frequencies below 50 cm-' show some anharmonicity. Magnitudes of displacements of atoms are mainly determined by the modes with frequencies below 30 cm-'. These very-low-frequency modes contribute significantly to the entropy of the system. The dynamic structure of the globular protein is described as a superposition of harmonic high-frequency motions and coupled anharmonic lowfrequency motions of collective variables corresponding to the normal modes of vibration.
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