We present an experiment in which spontaneous parametric down-conversion is suppressed or enhanced by interference with two photons from a classical local-oscillator field. When there is destructive interference, any photon pairs from the laser field undergo sum-frequency generation and are removed from the beam. In this sense, there is an effective two-photon nonlinearity which is significant even when only two photons are present at a time; the nonlinearity is observable at the quantum level because it is enhanced through the mediation of the strong classical field, which acts as a spectator. Although the scheme is accompanied by inevitable quantum noise, it functions essentially as an absorptive two-photon optical switch.