The method of Pariser and Parr, and Pople, is used to examine various effects of the interaction between two ethylene molecules. Changes in absorption frequencies and oscillator strengths, van der Waals energies due to the dispersion force, and time constants for resonance transfer of excitation energy are calculated for several relative positions of the two molecules. The asymptotic expressions ordinarily used in the theory of such effects are compared with calculated values for several intermolecular distances. Although the absorption frequency shift can have either sign, depending on the angular displacement of one molecule with respect to the axis of the other, the intensity shift is always of the same sign (hypochromic). These effects fall off as the inverse third and sixth powers, respectively, of the intermolecular distance, and should be of greatest importance for near-neighbour molecules, for which the asymptotic formulas cannot be expected to be quantitatively valid.