Presbyopic contact lens patients with monocular corrections (monovision) see clearly at all distances by virtue of an interocular suppression of anisometropic blur that occurs regionally between corresponding retinal areas. This suppression fails to occur with small high-contrast targets viewed under low luminance conditions. The effect of target size and contrast upon interocular suppression of blur was quantified by reducing contrast of a bright test spot, viewed binocularly while wearing various plus lenses monocularly, until the out-of-focus image was suppressed. The strength of interocular suppression was equivalent when the plus lens was before either eye. However, after subjects wore a plus lens over their nonsighting eye for one day, interocular suppression of blur became enhanced when the nonsighting eye was blurred, and it became reduced when the sighting eye was blurred. Successful monovision subjects suppressed blur at higher contrast levels than did unsuccessful subjects. These results suggest a possible clinical test for quantifying adaptation to monovision.