Abstract: Experiments showed that the antagonistic substance of B-903 strain still preserved it's inhibitory action to pathogenic fungi when autoclaved at 121℃, (1 4Pa/cm²) for 20 min in acid solution. However, this substance was not stable when treated by the same temperature at pH≥7.0. The antagonistic substance in acid solution (pH=4) could perserve it's inhibiting function for more than 6 monthes but only for 2 monthes in an alkaline solution. This substance could be precipitated and isolated in an strong acid solution and dissolved again in alkaline solution.