We have calculated the static and dynamic susceptibility of mixed-valence impurities in a metallic host. We use a simple model consisting of two stable multiplets with angular momenta J1 and J2, that correspond to the 4fn+1 and 4fn configurations, respectively, which are hybridized through the conduction-electron states. Two methods of calculation have been employed. For the static quantities we ... [Show full abstract] used an equation-of-motion method in connection with the fluctuation-dissipation theorem and the Brillouin-Wigner approach. The dynamic susceptibility is expressed in terms of a relaxation function N(z), which is calculated within a mode-mode coupling approach and within the Brillouin-Wigner method. The results obtained by the different methods are compared and applied to the cases of Ce, Nd, and Tm impurities.