In 15 cats, cortical area AI was defined by its frequency organization, and cells within that field were tested for sensitivity to interaural intensity differences (IIDs) using sealed stimulus delivery systems. Of 39 cells tested quantitatively, 26 were sensitive to IIDs. In 70% of cases, sensitivity to IIDs reflected suppressive binaural interactions, and was manifested as a sigmoidal relation of spike count to IID. For 8 other cells, facilitative binaural interactions generated unit sensitivity to IIDS; three of these neurons demonstrated nonmonotonic dependency of spike count on IID, with peak firing rates at or near 0 dB IID. Analysis of spike count versus IID functions in terms of the auditory azimuths known to generate the IIDs used revealed that the majority of cells were most sensitive to IIDs associated with azimuths in the contralateral sound field. These data are compatible with other evidence on the sensitivity of cortical and brainstem cells to binaural sound localization cues, and suggest that each side of the auditory brain is independently capable of localizing sound sources in the contralateral field.