In the visual masking effect, metacontrast, the perceived brightness of a flash of light is reduced by the presentation of a second flash in an adjacent region of the visual field. A metacontrast masking interaction between different colour mechanisms is examined in the present study. The spectral compositions and intensities of the stimuli are chosen so that the first (test) flash preferentially stimulates the blue-sensitive colour mechanism and the second (masking) flash preferentially stimulates the red-sensitive colour mechanism. Elevation in test-flash threshold is measured as a function of delay in presentation of the masking flash and an interaction effect shown to occur at both positive and negative masking-flash delays.