Beall, Francis A. (U.S. Army Chemical Corps, Frederick, Md.), Martha J. Taylor, and Curtis B. Thorne. Rapid lethal effect in rats of a third component found upon fractionating the toxin of Bacillus anthracis. J. Bacteriol. 83:1274–1280. 1962.—Rats were found to be more susceptible to the lethal effect of toxin produced by Bacillus anthracis in vitro than were several species considered less resistant to anthrax. Rats were killed much faster by less toxin per gram of body weight than were mice. Guinea pigs survived doses of toxin that killed rats. Intravenous injection of Fischer 344 rats is a rapid test for lethal activity, which facilitates the demonstration of two components, different from protective antigen, in toxin. One of these, a lethal factor, was separated from the other component, which causes cutaneous edema in the guinea pig. The latter component was not necessary for lethal effect. Neither of these factors was active unless combined with protective antigen. Although the guinea pig skin reaction has been used routinely to assay the toxicity of samples, the present results show that this test does not assay the lethal component.