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Superconducting tunnel junctions have been used as the nonlinear element for mixing at a signal frequency of 115 GHz. The experimental results are compared with predictions of a theoretical analysis based on the quantum theory of mixing of J. R. Tucker. Qualitative agreement is obtained and suggestions are made for quantitative reconciliation. The junctions were small area (≡0.4mu m2) with normal resistances of 60 to 100 omega and capacitance approximately 20 fF. Measured sensitivity (T/ sup SSBMXR=62 K, Lc=7.6 dB) implies receiver noise temperatures superior to the best receivers now in use at this frequency.