1. Responses of single auditory nerve fibers to combinations of noise and tone were obtained. The results were found to depend on the relative effectiveness of each stimulus when presented alone. 2. When the response rate to one stimulus presented alone was considerably greater than the response rate to the other stimulus presented alone, the more effective stimulus dominated the responses when the two stimuli were combined. The more effective stimulus captured the response of the neuron. Thus, intense noise was found to mask responses to weaker tones, and intense tones were found to mask responses to weaker noise. This masking of the weaker stimulus is thought to enhance the signal-to-noise ratio of the most prominent response component. 3. When the two stimuli had similar effectiveness, complex interactions occurred. When the tone was near best (characteristic) frequency, partial summation effects occured. The tone partially suppressed the responses to the noise if other frequencies were used. Tones above best frequency caused particularly powerful suppression. 4. The bandwidth of the noise was varied somewhat. While bandwidth affected the effectiveness of the noise, it did not affect the types of interactions observed. 5. For a neuron which was essentially silent in the absence of acoustic stimuli, adding a weak level of noise lowered the threshold of responsiveness to the tone.