The body portion (A) of a small portablesterilizer has a face panel (46) against which a door (B)is selectively closed. The face panel defines an accessopening for a sterilization chamber (10) which receives acassette (C), an access opening for an anti-microbialconcentrate chamber (20) which receives a powdered or othersterilant concentrate, and an outlet opening (50) from amicrobe filter which filters microbes from incoming rinsewater. The face plate and the door define fluid flowchannels (48, 52) therebetween for selectively directingsterilant solutions and rinse solutions among theanti-microbial concentrate chamber, the sterilization chamber,and the microbial filter. The cassette is configured toassure that it is inserted into the sterilization chamberwith a unique orientation such that its fluid inletapertures (114) and outlet apertures (124) are atpreselected locations. The door includes generallyU-shaped projections (116) which abut an outer surface of thecassette in the sterilization chamber partially surroundingthe fluid inlet apertures. The U-shaped projections assurethat the cassette is seated in the sterilization chamberand provide a well which directs the circulating fluidsinto the cassette inlet apertures. A pair of gaskets (56,86) surround the active portion of the face panel. Avacuum pump (90) selectively draws a vacuum in an annularregion (88) between the gaskets, which vacuum locks thedoor against the face plate in a sealed, closed position.To assure a fluid tight seal, the gaskets are O-rings whichare pressed by the vacuum into V-shaped grooves (82, 84).