Specific anthrax antigens were demonstrated in the blood of animals dying from anthrax. These antigens, which appear in the blood at the time when organisms are first detected and whose concentration continues to increase as the number of organisms increases, do not elicit a strong antibody response. The in vivo-produced toxin differs from the in vitro in killing more rapidly and being more difficult to detect. The in vivo toxin exists as an aggregate whose biological and serological activity depends upon its particular composition or configuration, or both.