The high-frequency elastic properties of rat-tail tendon collagen have been investigated by means of Brillouin (inelastic) light scattering. Longitudinally and transversely polarised elastic waves of frequency about 10¹⁰ Hz have been observed propagating at various angles to the fibre axis of stretched, partially dried tendon. Assuming that the elastic properties of tendon are transversely isotropic, these measurements enable the five elastic constants for such a system to be determined. In particular the ratio of the Young's modulus for strain parallel to the axis to that for strain perpendicular to the axis (E_∥/E_⟂) is found to be 1.43 and the ratio of the shear modulus to E_∥ is 0.28. In wet collagen only the longitudinal branch has been observed and in this case the ratio E_∥/E_⟂ increases to 1.82. The absolute value for E_∥ in dry collagen is 11.9 GN m⁻² reducing to 5.1 GN m⁻² in wet collagen. An interpretation of these results in terms of the expected vibrations of the collagen molecular assembly is given. Possible applications to the determination of the mechanical properties of collagen composite materials such as bone are discussed as well as some measurements on silk and α- and β-keratins, which are fibrous proteins of different molecular conformation to collagen.