A method of producing a highly porous melt spun fibrous tube for use as a separation medium comprising melt spinning a blend containing from 30 to 70 parts by weight of a first polymeric component and from 70 to 30 parts by weight of a second polymeric component to form a tube having a wall consisting of interpenetrating networks of the two polymeric components and, using a suitable solvent, leaching out of the tube wall one of the two polymer networks so producing a tube having a wall comprising the other polymer network.