The author calculates the lowest-order conduction electron vertex function Gamma of an Anderson lattice in the Kondo regime. There is a strong repulsion between a pair of opposite-spin electrons due to scattering from a single magnetic ion. The lowest-order attractive interaction is found to be between a pair of parallel-spin electrons and is due to scattering from two magnetic ions. The distance dependence is R^-2 instead of R^-3 as the usual RKKY interaction, so this interaction is longer-range.