Cellulose acetate membrane filter strips are used as a supporting medium for zone electrophoresis. Their main advantages are their chemical purity, minimal absorption and hence absence of tailing; very neat separation and complete transparency both to visible and U.V. light, when cleared by immersion in a suitable fluid. The cellulose acetate membrane filter strips are manufactured by Messrs. Courtaulds of Coventry and are obtainable from Oxo Ltd. (Med. Dept.), London.