The body portion (A) of a small portable sterilizer has a face panel (46) against which a door (B) is selectively closed. The face panel defines an access opening for a sterilization chamber (10) which receives a cassette (C), an access opening for an anti-microbial concentrate chamber (20) which receives a powdered or other sterilant concentrate, and an outlet opening (50) from a microbe filter which filters microbes from incoming rinse water. The face plate and the door define fluid flow channels (48, 52) therebetween for selectively directing sterilant solutions and rinse solutions among the anti-microbial concentrate chamber, the sterilization chamber, and the microbial filter. The cassette is configured to assure that it is inserted into the sterilization chamber with a unique orientation such that its fluid inlet apertures (114) and outlet apertures (124) are at preselected locations. The door includes generally U-shaped projections (116) which abut an outer surface of the cassette in the sterilization chamber partially surrounding the fluid inlet apertures. The U-shaped projections assure that the cassette is seated in the sterilization chamber and provide a well which directs the circulating fluids into the cassette inlet apertures. A pair of gaskets (56, 86) surround the active portion of the face panel. A vacuum pump (90) selectively draws a vacuum in an annular region (88) between the gaskets, which vacuum locks the door against the face plate in a sealed, closed position. To assure a fluid tight seal, the gaskets are O-rings which are pressed by the vacuum into V-shaped grooves (82, 84).