One method of providing multiple-access operation in a satellite communication system involves the transmission of many frequency-modulated carriers through a common broadband travelling-wave-tube (t.w.t.) amplifier in the satellite. Owing to the necessity to operate near the saturated output power of the t.w.t., amplitude nonlinearity and a.m./p.m. conversion will be significant factors in determining the overall system performance. Some results of multiple-carrier measurements using a t.w.t. of the type used in a satellite communication transponder are given in the paper. It has been shown that the performance is essentially determined by the third-order f1+f2−f3 type products for drives up to saturation. The agreement between theory and measurement is such that the carrier-to-total intermodulation-power ratio as a function of t.w.t. output power backoff can be derived from the measurement of a typical f1+f2−f3 type product when four carriers are transmitted through the t.w.t.Applying these results to the proposed global satellite Intelsat III, it is shown that at the optimum backoff of about −2dB the satellite telephone capacity, when operating to earth stations having a figure of merit of 39.3dB and a rain margin of 4dB, will be about 1300 telephone circuits.It is also shown that the threshold point of a frequency demodulator due to intermodulation products occurs at a carrier/intermodulation power ratio of +10dB, similar to that obtained when the noise is of thermal origin.