Abstract: A filter unit comprises a plastics frame into which a sheet of filter medium is insert moulded. Portions of the frame form apertures and other portions allow fluid access to and from the opposed surfaces of the filter medium when the frame is covered by a flat surface. The frames also form portions of an integral capsule enclosing the medium. A number of such filter units can be stacked together in various configurations so that the apertures form inlets and outlets for fluid so that, in some configurations, fluid can be passed through the filter units in parallel while, in other configurations, fluid can be passed through filter units in succession. Filters of any convenient size can be produced by increasing the number of filter units in a stack. The filters are bi-directional. The frames can be of polypropylene connected to hydrophilic microporous nylon membrane by injection moulding, which has been found to give a filter unit that can be integrity tested.