An explicit exact analytic expression for the energy of the electrostatic interaction between an ion-impenetrable sphere (hard sphere) with constant surface charge density and an ion-penetrable sphere (soft sphere) is derived on the basis of the linearized Poisson-Boltzmann equation without recourse to Derjaguin’s approximation. The influence of the internal fields induced within the hard sphere, that is, the polarization effect of the hard sphere is taken into account, which has been ignored in our previous papers [J. Colloid Inferface Sci. 157, 504 (1993); Colloid Polym. Sci. 271, 1191 (1993)]. It is shown that the next-order curvature correction to Derjaguin’s approximation, which arises from the effect of the internal induced fields, is of the order of 1/root kappa a instead of 1/kappa a (where kappa is the Debye-Huckel parameter and a is the sphere radius).