With the experimental apparatus designed and made available by Nicholes, we evaluated the bacterial filtration efficienty (B.F.E.) of the non-woven fabric and cotton cloth masks. The apparatus was supplied by Nicholes (U.S. Military Specification 36954 C mask, surgical, disposable). The study presented here was performed as a round robin test with Nicholes. By using this apparatus, comparison was made as to the B.F.E. of the six different kinds of surgical face mask before and after prolonged use. The result was obtained that the disposable mask made of glass fiber mat combined with non-woven fabric proved to be the highest in performance with a B.F.E. of 98.1-99.4%. It is useful both in preventing hospital infection and in general clinical practice. The B.F.E. of the conventional cotton cloth masks is not only lower but variable over a wide range of 43.1-93.6%.