We have studied the infrared reflectivity of normal and superconducting La2-xSrxCuO4 up to 4000 cm-1. For x=0.15 we observe an electronic excitation at about 60 meV (450 cm-1), and, at essentially the same energy, an optic phonon. As we move away from the optimal high critical temperature (Tc) value of x=0.15, we find that these features become much weaker. We propose that this 60-meV electronic excitation and the phonon in resonance with it provide the mediating interaction which gives rise to superconductivity at such high temperatures in this class of materials. © 1987 The American Physical Society.