The full use of the existing silicon irradiation facilities in the Harwell Material — Testing reactors over the last three years has encouraged the installation and commissioning of a new irradiation facility in the PLUTO reactor. This paper includes a description of the facility which uses a horizontal hole positioned tangentially to the reactor core. Included are some details of nuclear parameters and the mode of operation. It is conservatively estimated that the facility in full-time operation has a capacity in excess of 20 tonnes per annum.