Abstract: A device for separating molecular components from liquids. A multi-layer microporous membrane assembly is located within the housing (1) with edges of each membrane within the assembly lying adjacent to a barrier. Means (5,6) are provided for substantially preventing flow between the membrane edges and the barrier, so that flow is through the membranes in series. Inlet means (2) direct incoming liquid on to a first surface of the membrane assembly, and outlet means (3) gather and direct outgoing liquid from a second surface of the membrane assembly. The membrane material of the assembly has a ligand or an immobilized enzyme bound thereto for effecting chromatographic separation or an immobilized enzyme reaction.