Two nonidentical two-level atoms are separated by a fixed distance R. The probabilities of the atoms being excited are calculated for both the A\ensuremath{\cdot}p interaction and the E\ensuremath{\cdot}r interaction in the electric dipole approximation. For the A\ensuremath{\cdot}p interaction the second atom has a nonvanishing probability of being excited for times t<R/c, and so the interaction is not retarded. For the E\ensuremath{\cdot}r interaction the second atom has zero probability of being excited for times t<R/c, and so the interaction is completely retarded. Reasons for the difference are discussed.