Flashed-field sensitivities measured with a long-wavelength probe have a red/gree shape when stimuli are presented on a steady background but resemble the L-cone absorption spectrum in the dark. We previously speculated that this could be due to concealed color opponency; receptive-field surrounds, fed by M cones, are eliminated under dark adaptation. The current study tested this hypothesis with a sensitization paradigm. Thresholds were measured for a 640-nm probe presented on flashes of two sizes. Data were obtained in the dark and on a 4.0-log₁₀ td steady background. The results suggest a model in which the L cones comprise a center mechanism and both M and L cones a weaker inhibitory surround. Dark adaptation eliminates only the M component of the surround.