Toxic substance was isolated from the French presure cell extract of Pasteurella multocida avian type through ammonium sulfate fractionation, hydrochloric acid precipitation, Sephadex gel filtration, manganese chloride precipitation, calcium phosphate gel chromatography and zone electrophoresis. The substance was shown to be a single component by ultracentrifugation and agar diffusion test. The sedimentation constant in M/15 phosphate puffer (pH 7.0, i=0.16) for the toxin is Sw20=2.99×103. Total nitrogen of this substance was 16.3% ond showed the absorbtion of maximum in 280mμ, while negative in sugar and nucleic acid reaction, thus it was to be protein.