Abstract: A hollow fiber separation device such as an oxygenator for blood which comprises a core having a bundle of hollow tubular fibers circumferentially wrapped around the core for separation of materials. A housing encloses the bundle and first and second manifold systems respectively provide separated flow through the bundle in contact with exterior surfaces of the hollow fibers and the bores of the hollow fibers in first and second flow paths. A space extends longitudinally through the bundle exposing open ends of the fibers at opposed sides of the space, regions of the bundle are adjacent to the space being potted to prevent leakage around to the fibers. The second manifold system includes an inlet and outlet conduit, each being in flow communication with the space. Partition means in the space isolates fluid flow between the inlet conduit in one of the opposed sides from fluid flow between the outlet conduit in the other of opposed sides.