SUMMARY: Factor III of anthrax toxin, which increases the lethality of mixtures of factors I and II for mice and decreases their capacity to produce oedema in the skin of rabbits, has been purified. The final preparation showed a single peak in the ultracentrifuge and a single band on paper electro-phoresis. but it might still contain more than one serological component. The final preparation was a protein (N = 15.1%) containing all the usual amino acids but no carbohydrate, phosphorus, lipid or ash.