Measurements of the permeability coefficients of various compounds up-to a molecular weight of about 70,000 have shown clearly that membranes prepared from hydrolyzed polyacrylonitrile are about 10 times more permeable than those made of poly(2-hydroxyethyl) methacrylate. The higher permeability is probably due more to the higher water content (about 75%) than to the type of network. The high mechanical strength of the membranes and their good permeability to compounds possessing a comparatively high molecular weight seem to designate the material for some new applications in medicine.