The single-crystal elastic properties of a natural, grossular-rich garnet solid solution have been measured at ambient conditions using Brillouin spectroscopy. This garnet sample is unusual for a grossular in that it contains appreciable pyrope and almandine components, with a composition of CR48.4Py27.6Alm23.4Sp0.6. It is thus suitable for investigating the variation of elastic properties with composition in Ca-rich garnet solid solutions that are stable only at high pressure. The values obtained for the bulk modulus, K=170.4(6), and for the shear modulus, μ=101.6(3), of the garnet examined in this study, vary by less than 1% from those obtained from a weighted sum of the end-member moduli. This suggests that any composition in the grossular-pyrope-almandine system, which comprise the majority of upper mantle garnets, is accurately estimated with this simple approach. More complex elasticity-composition variations observed for the grossular-andradite garnets may be specific to garnet solid solutions where the cation substitutions occur at six-coordinated crystallographic sites. ¿ American Geophysical Union 1989