A description is given of the high-voltage X-ray tube and one-million-volt d.c. generator installed in the Mozelle Sassoon X-ray Therapy Department of St. Bartholomew's Hospital. The building contains a treatment room located between two generator rooms. The central portion of the X-ray tube, from which the X-ray beam emerges, traverses the treatment room and projects into each generator room. The generator rooms each house 500-kV d.c. generators. The X-ray tube and thermionic rectifiers used in the d.c. generators are evacuated continuously by oil diffusion pumps, and the whole apparatus is readily demountable. Removable filament assemblies are fitted to the rectifiers and facilitate rapid replacement of filaments; and a special cathode with 6 interchangeable filaments has been developed for the X-ray tube to avoid frequent admission of air into the tube during filament replacements. The tube is now used continuously at 1 000 kV and has been operated experimentally at 1 100 kV. Several structural changes have been, made for reasons stated later, and the performance of the tube is fully described. Curves relating X-ray output with voltage and filtration by metallic niters are given, and a radiograph as included to illustrate the capabilities of the tube if applied to industrial radiography. Comparisons are made between this tube and high-voltage tubes operating in other parts of the world.