Abstract: A semi-permeable membrane structure that is particularly adapted for plasmapheresis includes a frame having two tubes each forming an internal conduit opening inside the frame, and a membrane on either side of the frame, each membrane being mounted with its periphery disposed on the frame. Two external rigid plates each having a suitable support for a membrane on its internal face clamp the frame and membrane between themselves. Appropriate structure is provided for keeping the two rigid plates fixed to one another and thereby maintaining the spatial relationship of the frame, membranes and plates.