Rejection of local oscillator harmonic response is provided in a mixing circuit with a pair of harmonic gating switches serially connected to the outputs of a balanced differential switching mixer and controlled by a gate clock signal having twice the frequency of a local oscillator signal controlling the switching mixer. An aperture or duty cycle of the gate clock signal determines which harmonic is rejected or suppressed, which is preferably a third and/or fifth harmonic since response of the balanced differential switching mixer to even harmonics is negligible. The resulting simple, efficient circuit is readily integrated directly into a phase-alternating mixer structure for a chopper-direct-conversion radio.