A new and original type of chemical test using iron compounds was applied for the determinations of Bacillus anthracis and of its pathogenicity. The medium used was a modified North Gelatine Agar. The 2 procedures, the direct and indirect, for obtaining the reaction give positive and negative results. Virulent strains of B. anthracis give positive direct and indirect reactions, avirulent strains of B. anthracis give positive indirect and negative direct reactions, B. sub-tilis and other spore-bearing bacilli tested give negative direct and indirect reactions. The strength of the reaction varies directly with the virulence of the strain of B. anthracis. In the North medium, a substance seemingfy derived from its gelatin portion gives a red-purple color reaction with ferric chloride. B. anthracis has been the only organism to give the positive reactions. Some bacteria which prevent anthrax when they are simultaneously injected into mice, guinea pigs, and rabbits with B. anthracis, have also shown similar antagonistic action on this reaction.