Abstract: A filter assembly for clean air installations includes a block of high efficiency particulate air (HEPA) filter material adhesively bonded and sealed within an imperforate rigid thin-walled side frame which extends axially beyond the inlet surface of the filter block and thus provides an integral plenum chamber volume as part of a self-contained filter subassembly. The side frame has peripheral cover and supporting flanges at its inlet and outlet ends and can thus be conveniently assembled with and removed from a reusable cover member and cover gasket for the plenum chamber and a reusable perforated outlet grill and one or more peripheral outlet sealing gaskets, all of which constitute parts of a complete filter assembly which can be readily supported by a standard or special ceiling or wall grid or frame assembly.