A novel type of standing wave linear accelerator structure has been developed at Varian Associates, Inc. This accelerator is an interlaced combination of two side-cavity-coupled standing wave substructures. An S-band prototype accelerator guide with two identical substructures and total accelerating length of 10 cm has been built and successfully tested with a 2-MW magnetron. Significant improvement in maximum permissible accelerating gradient was achieved. Output electron beam energy of 4 MeV and corresponding accelerating gradient of 40 MeV/m were experimentally demonstrated. The stunt impedance of the structure corresponds to 83 MΩ/m.