Puziss, Milton (Fort Detrick, Frederick, Md.) and George G. Wright. Studies on immunity in anthrax. X. Gel-adsorbed protective antigen for immunization of man. J. Bacteriol. 85:230–236. 1963.—Protective antigen in filtrates of anaerobic cultures of Bacillus anthracis was not readily precipitable by alum or other aluminum salts, but was adsorbed onto a preformed aluminum hydroxide gel under specific conditions. The adsorbed product was highly effective in immunizing rabbits, guinea pigs, and mice against challenge with virulent B. anthracis spores. Gel-adsorbed anaerobic antigens preserved with 1:10,000 thimerosal were unstable on storage at 4 C; replacement of this preservative with 1:40,000 benzethonium chloride produced a more stable product. Addition of 0.0009% formalin further increased the stability during accelerated aging at 37 C. The final product had high protective activity in animals and was well tolerated in man.