A cassette for the sterilization and storage of dental or medical instruments includes a lower tray and an upper cover hingeably joined together for pivotal movement between an open position and a closed position. Both the tray and the cover have instrument positioning racks mounted therein to position the instruments in side-by-side relation within the tray and the cover. The cover is provided with an instrument retainer for simultaneously retaining the instruments in the positioning racks in the tray and the cover when the cassette is in the closed position. The instrument retainer includes an instrument retaining bar pivotally mounted at one end thereof within the cover and releasably fastened at the other end thereof to the cover. A resilient retaining member is mounted on the retaining bar and transversely pressingly contacts the instruments to retain them in the positioning racks. The instrument retainer is removable so that it can be adjusted between a first position and a second, inverted position to permit the storage of larger sizes of instruments within the cassette.