Anthrax toxin was present in the blood obtained from rhesus monkeys at the terminal stage of anthrax and was also present in increasing amounts in the lymph as death approached. When sterile toxin produced in vitro was injected in sufficient amounts, monkeys died unless they were injected with specific antiserum. Death with symptoms of toxemia occurred within 2 hours after injection of 10(11) spores, whereas 10(10) spores or less required 20 or more hours of growth in vivo to cause death.