An investigation of interference between angle-modulated systems carrying multichannel telephony, which is directly applicable to existing satellite and terrestrial radio-relay systems, has been undertaken. This study includes a general solution for arbitrary narrow-band interference into an angle-modulated system with arbitrary modulation. The algorithm has been implemented on a digital computer to provide calculations of interference between two FDM/FM systems. The analysis is valid for all modulation indices, and may be applied to any PM, FM, or preemphasized FM baseband. A comparison of calculated results and experimental measurements shows good agreement. An extensive investigation, covering a wide range of modulation indices and basebands, has been conducted for the particular case of preemphasized FDM/FM signals. Wherever possible, a generalized presentation of interference results is given. Interleaving criteria are also presented for certain cases of interference involving two identical signals.