Responses of single neurons and neuron clusters were studied at short intervals along penetrations into the high-frequency (4-25 kHz) representation of A1. The monaural and binaural sensitivities of neurons at best frequency were studied. With the exception of some neurons responsive only to binaural stimulation, the monaural responses of most were classified as contralateral dominant, ipsilateral dominant or equidominant. Binaural interactions of most neurons were classified as summation (binaural response size greater than the monaural response size) or suppression (dominant ear response size greater than the binaural response size). Most neurons arrayed in a column perpendicular to the cortical surface display the same aural dominance and binaural interaction. Summation columns occupy about two-thirds of the area sampled; suppression columns, about one-third. Within most suppression columns, the contralateral ear was dominant. Within summation columns, aural dominance varied. Summation columns appear to be composed of smaller columns differing in aural dominance. The sizes of binaural interaction columns vary considerably; some occupy several square millimeters of cortical surface. At least some binaural interaction columns occupy strips of cortex oriented orthogonal to isofrequency contours.