It having been shown in previous articles1 that the antigens of many bacteria and cells consist of pure fats, it seemed advisable to inquire whether a spore-bearing organism such as B. anthracis would yield a similar antigen. For this purpose there was selected a laboratory strain of anthrax bacillus, and a strain sent me from the Hygienic Laboratory at Washington. These strains were pure, the one a meager, the other an abundant, spore producer, and were easily raised in virulence for guinea-pigs and rabbits. While the method of isolation of fatty antigens has been given before, it will be briefly summarized.