During the past 50 years many attempts have been made to develop a method for preparing permeable collodion membranes of graded and uniform pore size. Such membranes would constitute an exceedingly important apparatus in biological investigation and should be especially useful in the study of filterable viruses.The method of preparation which has been most extensively used is that' of Bechhold (1-3), consisting of impregnating filter paper with solutions of nitrocellulose in glacial acetic acid, the permeability being regulated by the concentration of nitrocellulose used. These membranes have the serious defect of considerable variability in the size of pores; Elford (4) estimates that the largest pores in such a membrane may have a radius ten to twenty times greater than the average pore radius.A number of investigators have attempted to devise a method by which the use of filter paper could be eliminated and strong membranes of graded and uniform porosity be prepared. Bigelow (5), Walpole (6), Bartell and Carpenter (7), Hitchcock (8), Pierce (9), and others have controlled the permeability of membranes by varying the time allowed for evaporation of solvents from an etheralcohol solution of collodion. Eggerth (10) graded the membrane permeability by varying the alcohol content of the collodion solution. Brown (11,12) and also Nelson and Morgan (13) treated air-dried collodion membranes with different concentrations of alcohol in water, and so effected variation in permeability. Among non-volatile reagents added to ether-alcohol solutions of collodion for the purpose of altering membrane permeability are ethylene glycol (Pierce (9)), and glycerol (Schoep (14)). Asheshov (15) was the first to utilize volatile reagents, adding varying amounts of acetone to increase, or amyl alcohol to decrease, membrane permeability. In later work he recommends the use of acetic and formic ethers (16), and states that the results obtained by adding amyl alcohol are unreliable.Elford (4), using the same ingredients as Asheshov, found that acetone and 143