The absolute sensitivity of the binaural auditory system was studied by asking subjects to discriminate tones that were truly monaural from dichotic tones that sounded monaural because of an extremely large interaural difference of intensity. Thresholds for this discrimination across the range of frequencies from 250 to 4000 Hz were achieved when the tones in the weaker channel were in the region from −45 to −50 dB relative to tones in the stronger channel. Since the monaural reference tones, as well as the stronger signals in the dichotic conditions, were chosen to be 61 dB sensation level (SL), the weakest levels for which binaural interaction could be measured appear to have been on the order of 10 to 15 dB SL.