1. A method has been developed for testing the filtration efficiency of some filter materials. For each of the materials investigated — cotton wool, stillite and carbon — a suitable filter has been devised. 2. The filtered air was analyzed as to its germ content with the aid of a set of 3 capillary impingers. 3. The cotton wool filter gave on the whole satisfactory results provided that due attention was given to the packing of the filter and its sterilisation. Clear indications were obtained that the degree of the contamination of the air was of vital importance. 4. The stillite filter proved to have the advantage of combining a high filtration efficiency with a low resistance to the passing air. Also for the stillite filter a critical degree of contamination of the air was established; on surpassing this degree the filtration effect was endangered. 5. The carbon filter proved to be most efficient, but had a relatively low specific filtering capacity. It was found that the filtration result was depending on the height of the carbon column and on the velocity and the degree of contamination of the air. 6. It should be stressed that in all experiments artificially contaminated air was used, and that the number of germs present in the air to be filtered was in all cases many times larger than that usually occurring in normal air.