In the last three years the methods of immunizing calves against blackleg have been changed. The older methods of immunizing calves with small doses of virus, based on the work of Arloing, Thomas, and Wright have been largely replaced by sterile, soluble products derived from the blackleg bacillus. In 1917 Haslam and Franklin published a preliminary report (1) on the preparation and properties of natural blackleg aggressin. The publication did not have wide circulation, and several articles on the subject appearing subsequently (2) have failed to place the credit for the first practical use of a sterile blackleg immunizing agent in the United States. The work of Haslam and Franklin was the first demonstration of the practical value of sterile blackleg aggressin in actually controlling losses from blackleg on infected premises, although Roux (3) in 1888 and later Schobel (4) had shown by laboratory experiments that the sterile antigens possessed marked immunizing properties.