The slug holder is a mechanical device used to hold a collimating slug in the proper position in the beam of radiation of a linear accelerator. The design of the slug holder incorporates rigid construction for stability of the slug and repeatability of position. A gimbal adjustment is used to align the collimating slug in two angular and two lateral directions. The slug holder is attached to a plate designed to be inserted into the auxiliary tray holder of the particular linac being used. Precise construction enables collimating slugs to be inserted and removed with ease and extreme repeatability of position.