One, two, four, and eight simple tones were presented to listeners against a background of thermal noise. The masked thresholds for the single tones and the various combinations were determined, for different spacings of the tones. In the case of two tones, the improvement in threshold with respect to a single tone was slight or negligible unless the tones were within one critical band, when the improvement increased as the spacing decreased. In the case of four or eight tones all separated by more than a critical band, the improvement was slight (less than 3 db) or negligible, apparently depending on the combination of frequencies.