Abstract: An instrument sterilization cassette, into which instruments to be sterilized may be placed on an internal support rack structure, has a housing defined by an open-topped base portion to which a lid portion is pivotally secured. Spaced series of flow openings are formed through the top and bottom side walls of the lid and base portions and permit a vertical throughflow of sterilizing fluid through the cassette, and across the instruments supported therein, when the lid portion is closed. An external latch structure is operative to releasably hold the lid portion closed, and is secured to perforated upper and lower plates slidably supported on the inner side surfaces of the top and bottom side walls of the cassette. By pulling the latch structure outwardly from the housing, the plate members are slidably moved in the same direction and their openings are moved into registry with the top and bottom side wall flow openings to permit a throughflow of sterilizing fluid through the cassette.