‘GRADOCOL’ membranes of pore diameter less than 400 mµ change their characteristics when normally allowed to dry in air. These changes, which are irreversible, include a decrease in thickness and pore diameter when the membranes are re-equilibrated in water (Table 1). Membranes dried in this way and stored in the dark in contact with one another in a weighing bottle 2 in. diam. × 3 in. height tend to coalesce and decompose after long periods of storage. Membranes of low porosity are not easily re-wetted with water after being dry for several days. ‘Gradocol’ membranes may be dried by the method outlined below and stored for long periods without impairment of their characteristics.